Isaac made his first appearance in Omega, where he was shown working the late-night shift at Beacon Hills Cemetery, which his father owned. He was using a backhoe to dig a grave in preparation for a funeral when he stopped and looked in the rear-view mirror, gingerly touching a dark bruise that was developing around his right eye. Suddenly, he heard a sound nearby and turned on the light on the backhoe in an attempt to get a closer look. Just then, the figure rushed toward him and knocked the machine over, causing Isaac to fall into the grave he had just dug while the mysterious person, who, unbeknownst to Isaac, was a homeless Omega werewolf, began to eat one of the freshly-buried bodies in a grave.
Terrified, Isaac began to panic when he realized he was trapped in the grave due to the backhoe blocking him inside, until suddenly, the machine was pulled back into an upright position. Isaac looked up in surprise to find Derek Hale standing above him, and the older man offered him his hand before asking him if he needed help. The next day, Isaac stood next to his father, Mr. Lahey, while Sheriff Stilinski interviewed him about the grave robbery. Isaac admitted that the "thief" didn't take anything except for the body's liver. While being interviewed, Isaac saw Derek lurking in the shadows, which Sheriff noticed before asking him about his black eye. His father gave him a stern look, and Isaac ultimately lied and said he got it playing lacrosse, though it was obvious that Mr. Lahey had given it to him. Later that night, Isaac found Derek in his hideout at the Rail Depot and accepted Derek's offer of "the Bite."
In Shape Shifted, Isaac was eating dinner with his father, which seemed to be going well until his father asked him about his grades at school. Isaac evasively began with his good grades in English and French, but when his father continued to grill him about his chemistry grade, Isaac finally admitted that he had a D, but that there was still time to make it up. At first, Mr. Lahey responded kindly, conceding that he would have to punish Isaac but suggesting he could make it up to him by cleaning up the house. Isaac, relieved, agreed to this punishment, just as Mr. Lahey's demeanor turned from kind to enraged, smashing the dishes on the table and swiping the wreckage onto the floor before throwing a glass at him.
The glass missed Isaac's head and smashed against the wall behind him, but a piece of broken glass hit Isaac right under the eye, which he removed before angrily remarking that his father could have blinded him. Mr. Lahey initially blamed this on Isaac for his poor grades, but became distracted when Isaac removed the shard of glass, allowing his wound to heal right before his eyes and revealing that Isaac had successfully become a werewolf, though Mr. Lahey did not know this. When Isaac realized what happened, he panicked and ran out the door into the pouring rain, jumping onto his bike and riding away as his father chased after him. Mr. Lahey got into his car to catch up with Isaac, eventually parking on a street and walking into a dark alley where he thought Isaac had been cornered. However, it wasn't Isaac who was there, but instead a mysterious shapeshifter, who hissed at him before chasing him back to his car. Mr. Lahey desperately tried to hide inside the car, but creature ripped the door off before violently mauling him to death.
Isaac found out his father had been killed and immediately rushed back to Derek's hideout at the abandoned Rail Depot. Derek initially believed Isaac had lost control over his transformation as a neophyte werewolf, but Isaac explained that it was another shapeshifter. The next day, Scott McCall sensed another werewolf in the locker room at school and spent the duration of lacrosse practice smelling the various players to determine who it was, eventually learning it was Isaac while they were paired up to fight for the ball. Just as Scott saw Isaac's glowing gold eyes, Sheriff Stilinski, after learning from Jackson Whittemore that Mr. Lahey was abusive toward Isaac, arrived to arrest him as a suspect in his father's murder. Scott and Stiles Stilinski realized how dangerous this was when they realized that the full moon was that night, which led them to launch into a plan to both help Isaac by breaking him out of jail before he could harm anyone while also working to lock Scott up for the same reason.
To do this, Allison Argent eavesdropped on her father and grandfather talking about sending one of their fellow hunters, disguised as a Sheriff's deputy, to kill Isaac at the Sheriff's station with a syringe full of Wolfsbane, as they also believed him to be Mr. Lahey's killer. Allison slowed the hunter down by shooting him in the leg and the tire of his cruiser with an arrow while Derek and Stiles went to the Sheriff's station. There, Derek distracted the deputy at the front desk while Stiles attempted to steal the keys to the holding cells to free Isaac, only to find that the keys had already been snatched by someone else.
Isaac has just begun to shift in his jail cell when the hunter, with the keys in hand, arrived to kill him, but Stiles walked in at the same time, forcing the hunter to go after him instead to ensure Stiles couldn't out him to the real police officers. Suddenly, Isaac burst out of his cell and attacked the hunter, knocking him out before turning his sights on Stiles, who fearfully backed away from him until Derek arrived and roared him into submission. Shocked at the sight of Isaac whimpering in the corner and shifting back into a human, Stiles asked him how he managed to subdue Isaac, and Derek replied with a smirk that he was the Alpha.
In Ice Pick, when Scott went to the ice rink to try to talk Vernon Boyd out of taking the Bite from Derek, he was confronted by Derek, Isaac, and Erica Reyes, the latter of whom was Derek's most recently-turned Beta in his plan to rebuild the Hale Pack. Derek insisted that Scott at least take a census before claiming that the Bite would ruin Boyd's life, and Isaac remarked that, aside from being a fugitive, he believed being a werewolf was great. Erica and Isaac then attacked Scott, but since Scott had been a werewolf longer and had the experience to better use his powers and his fighting skills in tandem, he was easily able to defeat Erica and Isaac by knocking them both out and sliding them across the rink to land at Derek's feet.
In Abomination, Isaac and Erica were shown training with Derek at the pack's hideout in the Rail Depot, presumably due to their lackluster performance against Scott in the previous episode. While Boyd watched the practice, Isaac took advantage of the fact that he had access to his full powers (since he had already had his first full moon, unlike Boyd and Erica, who were bitten after the events of Shape Shifted) by performing numerous gymnastic feats in an effort to neutralize Derek. Unfortunately for Isaac, the Alpha defeated him every time and informed him that his attacks were too predictable. When Isaac asked if they could take a break so he could heal his broken bones, Derek approached him as if to comfort him, but at the last moment snapped his arm with his hand. As Isaac groaned in pain and cradled his newly-injured arm, Derek reminded them that the hunters would not be giving them breaks, and that between the threat of the new shapeshifter and the Argents, he needed to teach them everything he knew as fast as he could so they would have the skills necessary to survive.
In Venomous, Isaac, Erica and Derek kidnapped Jackson Whittemore in order to test him and see if he was the Kanima, as they were suspicious of anyone who had recently received the Bite and had seemingly not turned into a werewolf. Derek brought out a shard of glass from the previous episode that had leftover Kanima venom on it, believing that whoever the Kanima was would be immune to their own venom. Isaac held Jackson down as Derek dripped the venom into his mouth, which instantly paralyzed him. Believing that Jackson had passed the test and was not the Kanima, Derek and the others left the room, leaving only Isaac with the paralyzed Jackson. Isaac smiled smugly at Jackson and insisted that he needed him to do him a favor, taking advantage of the fact that Jackson was in a vulnerable position. It was later revealed that this favor involved having Jackson recant his previous statement to Sheriff Stilinski about Mr. Lahey being abusive to Isaac, which then forced Sheriff to take Isaac off of the list of suspects in his murder and allowed Isaac to return to his normal life and no longer be a fugitive.
Erica and Isaac were then given the task of performing the same test on Lydia Martin, who had been attacked and bitten by Peter Hale while he was still alive and the Alpha, and who had recently spent two days running around the woods naked as a result. They decide to perform the test in their chemistry class, where they were making sugar crystals in pairs that were rotated every so often. Isaac was first paired up with Stiles, who knew what Isaac was planning and threatened to turn him into a fur coat for Lydia if he did anything to harm her. Isaac admitted that he used to have a crush on Lydia, but when he asked her out their freshman year, Lydia rejected him due to Isaac not having a car, which made present-day Isaac decide he'd rather kill her instead. When the pairs were switched, Isaac was paired up with Lydia, and when their sugar crystals were finished, he surreptitiously coated the crystals in Kanima venom. Scott and Stiles tried to stop Lydia from eating it, but were too late; however, the Kanima venom had no effect on Lydia whatsoever, which led the Hale Pack to believe that she was the Kanima.
After school, Erica and Isaac rushed to find Lydia so they could kill her on Derek's orders, but when they got to the library, they found no one there but Matt Daehler, who had no idea where they were. Scott, Stiles, Allison, Jackson, and Lydia then all piled into the Jeep and went to the McCall House in order to hide from Derek, Isaac, Erica, and Boyd and protect the clueless Lydia from them. The Hale Pack stood outside of the house for a while before Isaac and Erica were sent inside to deal with the human high-schoolers. Isaac attempted to sneak up on Allison and Stiles, but was quickly defeated and knocked out by Scott while Allison incapacitated Erica by shooting at her with a Kanima venom-coated arrow.
Both groups were later stunned by the appearance of the Kanima crawling out of the upstairs window of the McCall House, and even more so when Lydia rushed out onto the porch and hysterically demanded to know what the hell was going on, revealing that Derek was wrong and that Jackson was indeed the Kanima. However, Derek and his pack would not figure this out for several more episodes due to them being unaware that Jackson was with them in the house. Derek then instructed Boyd to take the unconscious Isaac and paralyzed Erica back to the Rail Depot before the police officers that Scott and Stiles called arrived.
In Restraint, Isaac and Erica were ordered by Derek to get on the good side of Scott and Stiles, as he believed their help would be needed in order to identify and stop the Kanima. Isaac suggested that it might be best to leave the Kanima for the Argent hunters to handle, but Derek reminded him that he had seen the Argents' leader, Gerard Argent, acting strangely around it as though he knew something, and Derek didn't want to take the chance of Gerard using it against them. Isaac then worried aloud about the upcoming full moon, since Erica, Boyd, and himself would be chained up, leaving Derek alone and with no backup should the Argents come for them at that time.
In Raving, after learning for certain that Jackson was the Kanima, Derek and Isaac met Scott and Alan Deaton at the Beacon Hills Animal Clinic in order to work together to both stop and save Jackson, though Isaac made it clear that he didn't trust Scott before doing so. In the exam room, Isaac curiously looked through the various jars of herbs and other mystical substances, which caused Derek to roughly grab him by the wrist and tell him not to touch anything. Isaac then proceeded to rudely ask Deaton if he was some kind of witch, only for Deaton to drolly respond that he was only a veterinarian.
When discussing the Kanima and its master, Scott brought up the victims and the folklore that stated that the Kanima is meant to kill murderers, leading Derek to ask if this meant that Isaac's father was a murderer; in response, Isaac admitted that he wouldn't be surprised if this was the case. After Derek informed Deaton that the Kanima seemed practically invulnerable, since he had seen Chris Argent empty an entire clip into it without even slowing it down, Isaac suggested that perhaps they should be looking for an offensive plan rather than a defensive one. This led Deaton to get the idea to use mountain ash, a mystical barrier against the supernatural, to surround the location of the upcoming rave-- any humans would be able to come and go freely, but supernatural creatures, including the Kanima and its master, would be trapped inside, allowing them to finally learn the identity of the person using Jackson as a weapon of vengeance.
The next day at school, Isaac overheard Scott and Stiles discussing the fact that they still couldn't find tickets to the rave and rudely remarked, "How do you two losers even survive?" before roughing up a teammate and stealing two of his tickets, which he then gave to the boys. Later that night, Isaac arrived at the rave with Erica and immediately began looking around for Jackson. Scott caught up with him later and gave him a syringe-gun and a vial of ketamine, a tranquilizer, which he had been given by Deaton. Scott wasted no time instructing him on how to administer the drug to Jackson, but when he then Isaac to be careful, Isaac, believing that Scott was referring to Jackson, assured him that the needle probably wouldn't even hurt him. However, Scott clarified that he meant that he didn't want Isaac to get hurt, and Isaac was visibly shocked at the fact that Scott genuinely cared about him, something he hadn't felt from a person in a long time. It was this event that began Isaac and Scott's future close friendship.
He and Erica then sought out Jackson, eventually finding him on the dance floor, where the three tried to distract him by grinding with him in a flirtatious threesome of sorts. Unfortunately, Jackson's Kanima master, who was also at the rave, realized what they were up to and ordered Jackson to attack them with his venom-coated claws before then speaking through him, warning the two werewolves that the Kanima belonged to him. However, Isaac's exposure to the venom seemed to be minimal, because he was able to fight through it enough to grab the syringe and inject the ketamine into Jackson's neck. Isaac and Erica brought him to an empty room where they were joined by Stiles after he had successfully surrounded the building with a barrier of mountain ash. Isaac flicked out his claws to see if Jackson was fully unconscious, but before he could touch him, Jackson grabbed him by the arm and twisted his wrist until it snapped. Stiles demanded that Isaac not do that again out of fear he would be hurt further before the three began to question Jackson, who was temporarily paralyzed but not unconscious.
Jackson informed them that he was killing murderers who killed him, confusing Stiles, Erica, and Isaac before overcoming the ketamine and starting to transform into a Kanima. When Isaac admitted that he had already given Jackson the whole vial of ketamine, the three ran outside and pushed themselves against the door to keep him from escaping. However, Jackson simply broke through the corrugated steel wall next to them and runs free. Erica and Isaac rushed outside of the warehouse to find that they can't leave because of Stiles' mountain ash barrier, though they were able to pass after Derek forced Stiles to break it so he could save Scott.
In Party Guessed, Isaac, Erica, Boyd, and Derek gathered at their hideout in the Rail Depot to prepare for the full moon that night. Derek opened a trunk full of chains and other restraints to be used to chain up the newbie werewolves. Isaac noticed the triple-spiral inscribed on the lid of the trunk and asked about it, and Boyd explained that it was a triskele, which can symbolize different forms of trinities, such as past, present, and future, or mother, father, child. Derek was visibly impressed by Boyd's knowledge and explained that, to the Hale Family, it represents "Alpha, Beta, Omega," to remind them that werewolves can easily rise to one status or fall to another. Isaac brought up Scott and asked if he was an Omega, but Derek insisted that Scott was a member of their pack now, as he had finally joined them to help them defeat the Kanima.
That night, Derek brought out the restraints and began to chain them up inside one of the abandoned train cars at the hide out, and when Isaac saw a brutal-looking iron ring with screws drilled through it, he asked Derek why Erica was the one who would wear it around her head. Derek explained that it was because, as a woman, Erica would be able to withstand more pain than the rest of them, but mockingly offered to let Isaac wear one as well, which Isaac hastily denied. Isaac then helped hold Erica steady as Derek screwed the device onto Erica's head, wincing sympathetically as Erica cried out in pain.
Afterward, Derek began to chain an increasingly-agitated Isaac to one of the seats in the train car, where Isaac groaned in pain and asked Derek how he couldn't feel the full moon. Derek irritably replied that he felt every second of it, but that he has learned to control his transformation by finding an anchor and binding himself to it, using it to connect him to his humanity. When Isaac asked what his anchor was, Derek answered that it was anger, but that each person's anchor is different, which once again brought Isaac to the topic of Scott. Once Derek finished, he pulled on the chains to make sure they were secure and looked horrified when one of the chair legs came unbolted from the floor, indicating that Isaac could easily get out if he wanted. Behind him, Boyd and Erica had fully shifted into their werewolf forms and growled at Derek menacingly before eventually breaking through their restraints and attacking Derek.
Isaac, too, was able to escape his own bonds and jumped out the window, but he later returned and, much to Derek's surprise, helped him restrain the others again before allowing Derek to chain him up. While shackling Isaac's wrists to his seat, Derek remarked that Isaac had found an anchor, and Isaac confessed that it was the memory of his father. When Derek scoffed and reminded him that his father used to lock him in a deep freezer to punish him, Isaac replied that he didn't always act that way, suggesting he was a loving father before the death of his wife and firstborn son.
In Battlefield, Boyd and Erica, who were overwhelmed by the intense threats of the Argent hunters and the Kanima, made the decision to flee Beacon Hills in hopes of finding a new pack in a location that wasn't so dangerous. They also invited Isaac to come with them, though the young werewolf was so conflicted about whether or not to accept their offer that he went to the animal clinic to talk to Scott at work, hoping that the fellow werewolf could give him advice.
He found Scott and Alan Deaton treating a dog in the exam room, and Isaac wrinkled his nose in disgust, realizing that he, like Scott did shortly after being turned, could smell illness in animals and was even able to discern that the dog was dying of cancer. Deaton then asked Isaac if he had ever considered that he could use his powers to help others before taking his hand and gently setting it on the dog's side, which caused the blood vessels in Isaac's hands to darken as it traveled up his arm. Shocked, Isaac asked them what just happened, and Deaton informed him that he had taken some of the dog's pain, enough to take the edge off and possibly allow him to heal better. Isaac was so overwhelmed that his eyes filled with tears, and Scott assured him that he cried the first time he did it as well.
Afterward, Deaton left Isaac and Scott to talk privately, and Isaac admitted that Boyd and Erica asked him to leave town with them. Scott was curious as to why Isaac would tell him that, so Isaac confessed that he was hoping to get Scott's advice on the subject, as he saw Scott as someone who always wanted to do the right thing, though Scott insisted that he has no idea what he's doing most of the time. When Isaac asked Scott what he planned to do, Scott replied that he had to stay in Beacon Hills because there were too many people there who depended on him. Isaac sadly stated that the difference between them was that he didn't have anyone, and implied he was going to leave with Boyd and Erica before wishing him luck in the lacrosse game. Scott informed him that he wasn't planning on playing in the lacrosse game, since it seemed meaningless and insignificant compared to the larger issues at play. This response reminded Isaac that he had seen Jackson at practice the previous week, indicating that the Kanima would be playing in the game that night and leading Scott to realize he needed to be there to protect everyone from him.
That night, Gerard Argent, as principal of Beacon Hills High School, knew that Scott would likely try to interfere with his plans and forced Coach Finstock to keep Scott on the bench to keep him out of it, though he claimed the punishment was due to Scott's poor grades. Right as the game was about to start, Scott, seated on the bench, was pleasantly surprised when Isaac, in his lacrosse uniform and pads, sat down next to him, revealing he decided to stay to help Scott fight and win against Gerard.
However, when Gerard revealed that he planned to have Jackson kill someone at the end of the game if Scott didn't give him Derek, the two decided they needed a plan that would force Coach to put Scott on the field to prevent the bloodbath. Isaac ultimately decided to do this by purposely injuring their own players one after the other so Coach would have no choice but to bring in Scott to replace them. Unfortunately for them, while this plan ultimately worked, Gerard still realized what Isaac was doing and had Jackson tackle him, breaking Isaac's leg and paralyzing him in the process. When the paramedics brought Isaac inside for treatment, Gerard knocked them out before pulling out his broadsword, preparing to kill Isaac by cutting him in half as the werewolf desperately crawled away. Fortunately for Isaac, Scott appeared at the last moment to save his new friend, though Gerard escaped before he could be attacked.
In Master Plan, after the Beacon Hills Cyclones won the game, Jackson committed suicide by impaling himself with his claws, leading the Beacon Hills werewolves to think that Jackson purposely killed himself to prevent Gerard from using him to kill one of them. To make matters worse, Stiles had suddenly gone missing, leading Sheriff Stilinski to talk to Isaac and Scott in hopes that they might know what happened to him. Knowing that his abduction was related to the supernatural and wanting to keep Sheriff out of it, Scott suggested that Stiles had simply left early after being overwhelmed by the attention of being the game-winning player, before promising that he'd let him know if he heard from him.
Once he left, Scott and Isaac raided Stiles' locker to get his scent so they could go find him, but before they could leave, they were confronted by Derek and the recently-resurrected Peter Hale, who informed them they needed to talk. Isaac immediately asked Scott who Peter was, and Scott briefly explained that he is Derek's uncle who tried to kill them before they set him on fire and let Derek slash his throat. Once the introductions were out of the way, Peter revealed that he had a plan to save Jackson, and both he and Derek were stunned when Scott informed them that it was too late, since Jackson had just killed himself minutes earlier. Isaac wondered aloud why none of them seemed to be taking this as good news, but Peter responded that if Jackson was dead, then it was because Gerard wanted him to be, meaning that it was all part of his ultimate plan.
Just then, Scott received a phone call from his mother, Melissa McCall, a nurse at the hospital who had examined Jackson in the morgue and found something strange. When Isaac and Scott split up from Derek and Peter and arrived at the hospital, Melissa showed them that Jackson's body had covered itself in a clear cocoon made from the venom released from his claws. Jackson then began to twitch, suggesting he was being reborn, and at a loss for options, the two boys decided that they needed to bring Jackson with them to meet Derek and Peter halfway. While they were smuggling Jackson's body out of the hospital, they were caught by Chris Argent; however, instead of stopping them as they assumed he was there to do, Chris offered to help them, as he believed Gerard was manipulating Allison and was worried about her. He also pointed out that he knew that Scott could feel her slipping away from them as well.
The three men took Jackson's body and drove it to the Warehouse District to meet up with Derek and Peter, not knowing that they were walking into a trap set by Gerard and Allison, who had entered a dark period due to her grief from her mother's recent death. Allison shot Derek with an arrow, and Scott dragged him to safety before Isaac attempted to join in the fight, where he was easily defeated by Allison and her Chinese ring daggers. When it was revealed by Gerard that he was seeking the Bite from Derek, Isaac, using his newly-developed sense of smell, realized that Gerard was dying of cancer and wanted to become a werewolf to cure it. Isaac watched in horror as Scott was forced to make the paralyzed Derek give Gerard the Bite, with no one but Scott knowing that the young werewolf had switched out Gerard's cancer medication with pills full of mountain ash, which caused Gerard to reject the Bite almost as soon as he received it. After Lydia and Stiles arrived just in time for Lydia to help Jackson transform into the werewolf he was meant to be instead of a Kanima, the battle was finally over.
The next day, Isaac walked up to the Hale House with Derek and Peter, where they found a strange black symbol spray-painted onto the front door. When Isaac walked up to the front porch to examine it more closely, he asked Derek what it meant, and Peter grimly revealed that Derek had been so eager to rebuild the Hale Pack for a reason-- because as a powerful new Alpha and a Hale, he attracted the attention of the Alpha Pack, who now sought to add Derek to their ranks. As a result, the symbol was meant to act as a warning that they had already arrived.