Brian O'Conner let Dominic Toretto go at the end of the first Fast & Furious movie, instead of arresting him, because he respected him and realized he didn't deserve to be jailed. The characters, played by Paul Walker and Vin Diesel, respectively, were first introduced in 2001's The Fast and the Furious, which pinned Brian and Dom against each other considering the former was an undercover member of the LAPD. In the end, Brian let Dom walk free, but he wouldn't reveal the main reason why until the fourth movie in the franchise.
As an undercover detective, Brian was assigned to investigate a case that involved a series of truck hijackings. His findings led him to the local street racing scene where he first came in contact with Dom and his crew. Brian managed to infiltrate the group by befriending Dom and forming a relationship with Dom's sister, Mia Toretto (Jordana Brewster). Acting as an ally to the group, Brian investigated rival groups of street racers but quickly realized that Dom was behind the hijackings.
Before Dom emerged as the top suspect, Brian sent a SWAT team to the home of Johnny Tran (Rick Yune), the leader of another crew. Thinking that Dom's crew sold him out to the police, Tran viewed Dom as a threat so he and another member of his crew shot up the Toretto house. Dom and Brian happened to be outside, confronting one another about the truth. Jesse (Chad Lindberg) was killed from the drive-by shooting, which caused Dom and Brian to give chase. After taking out Tran, Dom and Brian competed in a quarter-mile race. Dom wrecked his car after crossing the finish line, but rather than pull the man out so he can be arrested, Brian gave him the keys to his Supra and allowed him to escape. Brian claimed that he owed Dom a "10-second car" from an earlier race, but that wouldn't be the only explanation.
Brian Revealed His Reasoning In 2009's Fast & Furious
Dom took the opportunity given to him by Brian and fled to Mexico. Meanwhile, Brian was charged with aiding and abetting so he was removed from the LAPD. Following his work in 2 Fast 2 Furious, he found his way into the FBI as an agent but didn't waste time in reuniting with Dom in Fast & Furious. The two teamed up to track down Arturo Braga (John Ortiz), a known drug trafficker. While doing so, Brian rekindled his romance with Mia but she was still frustrated by his two-sided actions seen in the first movie. She questioned why Brian decided to let Dom go to which he realized he wasn't entirely sure. He later explained that he respected Dom more than he did himself and referenced the man's moral code. It probably also had to do with the guilt that Brian felt due to his involvement in the events that led to Jesse's death.
It was clear to Brian from the very beginning that friends and family meant everything to Dom. Time and time again, Dom proved that he would put his life on the line to help others; Brian admired that in him and valued close relationships in the same way. As the Fast & Furious franchise grew, so did Dom and Brian's friendship. Brian was already a member of the Toretto family before he and Mia started a family. Even though Brian's role has diminished due to Walker's death, his bond with Dom will always be a bright spot of the film series.
