Jase arrives in Walford in search of his estranged son, Jay Brown. He flirts with a pregnant Honey Mitchell, angering her husband Billy Mitchell. When Billy finds Jase's letter regarding visits to a probation office, he tells the community that Jase is a criminal, causing them to shun him. When Jay discovers his father has been in prison, he reacts badly and is arrested for carving "LIAR" into a bus shelter, referring to Jase's lies about where he had been all his life. In wake of this, despite the initial animosity, Jase and Jay begin to bond.
Jase meets up with Terry Bates, leader of a gang of football hooligans, who looked after Jase when he was a teenager. It is revealed Jase's involvement with the gang, led to his imprisonment. He decides that he does not want to be part of the gang any longer, as he has Jay to look after. Terry is angry about this and makes various threats to get him to reconsider. Jase refuses and Terry's gang raid The Queen Victoria looking for him. Jase is dragged into the cellar and beaten. Meanwhile, a heavily pregnant Honey calls the police and steps in to stop Jase being attacked; however, in retaliation, she is assaulted by the gang, after being caught-up in a violent siege. She goes into premature labour and gives birth to her son, Will Mitchell. Enraged by the gang attack, Jase swears revenge on Terry and threatens to kill him to gain his freedom, but he is stopped by the intervention of Phil Mitchell, who calls the police; the gang is then arrested. Phil expects repayment for his help and forces Jase to assist him in his various vendettas.
Following a brief fling with Roxy Mitchell, Jase begins a more serious relationship with Dawn Swann, whom he later proposes to. He unknowingly begins working for May Wright, the woman who had planned to adopt Dawn's daughter, Summer. A violent confrontation occurs when May tries again to abscond with Summer and causes a gas explosion in Dawn's house; Jase saves Summer's life, catching her when she is dropped from a first floor window. With their house burnt down and no money, Dawn begins to fret over their financial future. Wanting to please Dawn, Jase takes a mysterious call saying he will do "one more job for him." The next day Jase brings home a bag full of money. Dawn then spends the money on various things including clothes and rent money for a new flat. Jase adopts Summer and his name is added to her birth certificate, after Dawn had left it blank when Summer was born. Dawn's demands continue, so Jase does more jobs for his associate. When he comes home beaten and bloodied, he is forced to admit to Dawn that he is working for Terry Bates once again. Dawn is initially furious, but despite realising that he is involved in criminal activity, she makes no attempt to stop him as she is desperate to keep the money and get the wedding of her dreams. Jase speaks of leaving Walford to escape Terry, but Dawn announces that they have set a date for their wedding, ensuring that Jase cannot leave.
Upon cancelling his stag night to collect £100,000 for Terry, Jase reluctantly takes Billy on as a getaway driver, revealing his intention to steal Terry's money. They agree to split the money 60/40, however upon arriving at the location of the job, there is no money, only a note stating that Terry and his men have Jay hostage and the job is a setup. When they arrive back at the flat in search of Jay, Jase and Billy are greeted by Terry and his men wielding baseball bats. Eventually, the confrontation between Jase, Terry and his men leads to violence; Jase and Billy manage to save Jay, but in the ensuing fight, Jase is brutally beaten up by Terry's men. To Billy's horror, he witnesses the assault; however, he is too frightened to help Jase and hides in the bathroom. After beating Jase up, Terry fatally stabs him. Meanwhile, Jay, hiding in the van, makes an immediate call to the police. The police arrive and arrest Terry and his men, however, Jase later dies of his injuries in hospital. Dawn learns of his death on their wedding day. Jase is last seen when Jay identifies his body in the mortuary.
He is later cremated on 7 October 2008, with Jay, Dawn, Darren Miller, Garry Hobbs, Minty Peterson and Billy attending the funeral. Several months later, Billy eventually admits to Jay what happened on the night Jase died after being blackmailed by Nick Cotton, who threatened to tell Jay about his cowardice. Viv Bates, Terry's wife, and his gang threaten Billy into staying away from the court case by kidnapping Jay for security measures. Billy saves him, and he finally testifies against Terry, which results in him receiving life imprisonment for Jase's murder.