According to Abstergo, Sean and the other recruits were part of what the Templars researchers called an "Ascendance Event"; multiple bloodlines who had been in contact with Pieces of Eden mysteriously converging at some point in history as if they were guided by an unidentified force.
During his high school years, Sean was a football player before being rendered a paraplegic by a drunk driver.
Recruited by Monroe
At some point, Sean heard about Monroe, a discreet It guy who was known for offering free Animus sessions to willing youngster. Sean used the Animus as a way to escape his wheelchair by experiencing the memories of one of his ancestors, an Irish farmer, walking again through him.
In 2016, Sean was contacted by Monroe who asked him to join a group of other teenagers who like him had Templar or Assassin ancestors, in order to find a Piece of Eden, the Trident. The group, which in addition to Sean was composed of Owen, Javier, Natalya and the siblings Grace and David, accepted after Monroe told them about the secret war and his wish to hide the artifact from both the belligerents.
Under the guidance of Monroe, Sean and the other teenagers successfully discovered the clues leading to the location of the artifact while exploring the memories of their ancestors during the New York City draft riots of 1863. In the case of Sean, his ancestor was New Yorker Tommy Greyling, a former Union soldier turned policeman who unknowingly witnessed the fight between the Templars and the Assassins for a part of the Trident. While reliving the memories of Greyling, Sean also witnessed his ancestor falling in love with singer Adelina Patti while protecting her during the riots.
However, as the team eventually discovered that the Trident prong was in the house of the late Ulysses S. Grant, a Templar strike team assaulted the warehouse where they were using the Animus. Sean and three other teenagers, Grace, David and Natalya, were captured and brought to The Aerie, where the Templars Isaiah and Victoria Bibeau tried to convince them to help their Order. At first reluctant, Sean was eventually seduced by the Templar ideals and decided to trust them and help Abstergo find the Trident.
Working for Abstergo
Later and now working with the Templars, Sean had to relive again the memories of Greyling, this time in 1872. Now a Pinkerton detective, Greyling went to London in order to catch a female Templar murderer and became involved with Assassins Evie Frye and Henry Green. Sean's initial synchronization seemed to indicate to Isaiah that Greyling failed to catch the woman. However, when Sean later discussed his Animus session with his friends, he realized that Greyling intercepted the female Templar on a steamship leaving London and went back into the Animus to prove it. His memories confirmed that the Templar, named Alice, destroyed the manuscript she possessed and committed suicide rather than be captured by Greyling. After Isaiah reluctantly thanked him for his intuition, Sean discussed with his friends how Greyling's memories showed him that the world wasn't as black and white as he thought.
Victoria later had Sean and his friends synchronize with different ancestors to find the remaining prongs, as they did not have Monroe's Animus core to narrow down the locations. Sean eagerly participated in memory research with the promise that Abstergo would be able to create a prosthetic to enable him to walk again. One day, David told Sean and the others that he wandered the Aerie during the night and overheard Isaiah placing a kill order on Owen and Javier.
Sean was later summoned to the conference room, where he overheard Victoria and Isaiah arguing over the latter using Sean and his friends. Involving himself in the argument, Sean insisted he could speak for himself. Despite Victoria's concerns over his addiction to the Animus, Sean became more concerned with Dr. Thomas Marshall promising that they could have a working prosthetic in six months to a year. Subsequently, Sean went to explore the memories of his Jomviking ancestor, Styrbjörn the Strong, who was on the path of the second prong.
Victoria and Isaiah later discover David's late night excursion and told Sean and his friends that they would be confined to quarters at night. When given the option to go home, Sean chose to stay. After the Templars had left the room, Sean blamed David for their troubles and warned him against further misdemeanor.
Later at night, Sean woke to find his friends at his door, claiming that the power had gone out and that a siren had sounded, the latter he hadn't heard. When his friends decided to look around, Sean refused to follow. Not long after, Isaiah entered his room to see if he was still there, before a Templar agent informed his leader that David and Natalya were gone. Sean subsequently went to Grace's side to comfort her.
After a little while, Sean and Grace were summoned to Isaiah's office, where they were surprised to see Owen, dressed as an Assassin and bound by the wrists. Isaiah told them that Owen, along with Javier and the Assassin Griffin, staged an assault on the Aerie, to which Owen claimed was a rescue mission, which Sean thought to be an insult as if he was helpless. Most of Sean's anger soon dissipated when Isaiah revealed his knowledge of Owen's obsession with proving his late father's innocence, claiming that the Assassins set him up for the robbery that he was imprisoned for. Owen refused to believe him and produced a DNA sample of his father's from after the robbery, demanding access to Helix so he could see the truth. Isaiah asked for him to wait until morning for the DNA to be decoded and afterwards Sean and Grace showed Owen to his room.
The next morning, Sean got into a debate with Owen over whose side was in the right. Sean defended the Templars, believing them different than Boss Tweed and Cudgel Cormac. Subsequently, Isaiah and Victoria entered the breakfast room to hurry them onto their simulations; Sean expressed hope that Owen would find what he was looking for in his simulation, before returning to his own simulation.
Later Victoria's office, Owen asked her if he could see Monroe, who was captured in an independent rescue attempt, having been promised by Isaiah, but Victoria claimed no knowledge of such a deal. Owen and Grace then deduce that Isaiah was going after the second prong, the former guessing it may be in China or Mongolia. After Victoria had left the room, Owen encouraged Grace to help him find where Monroe was being held. Sean chose to stay and agreed not to rat them out.
Later on, Sean confirmed that Styrbjörn indeed had his hands on the third prong, before being pulled out of the Animus by Isaiah, who told Sean to come with him.
- After they both relived their ancestors memories and feelings Greyling falling in love with Natalya's ancestor, Adelina Patti, Sean developed a crush for the girl.