White Heat

James Cagney as Cody Jarrett.
Arthur "Cody" Jarrett (James Cagney) is a ruthless, deranged criminal gang leader. Although married to Verna (Virginia Mayo), Cody is overly attached to his equally crooked and determined mother, "Ma" Jarrett (Margaret Wycherly), his only real confidante (Cody's father died in an insane asylum). Cody suffers from debilitating headaches, and Ma consoles himâeven sitting him on her lap and giving him a shot of whiskey with the toast, "Top of the world," an expression she uses more than once.
Cody and his gang rob a mail train in the High Sierra at the California border, resulting in the deaths of four members of the train crew as well as a member of Cody's gang, Zuckie (Ford Rainey). With the help of informants, the authorities close in on a motor court in Los Angeles where Cody, Verna and Ma are staying. Cody shoots and wounds US Treasury investigator Philip Evans (John Archer) and makes his escape. He then comes up with a schemeâto confess to a lesser crime committed in Springfield, Illinois, which an associate committed at the same time as the train robbery, thus providing him with a false alibi. He turns himself in and is sent back to Illinois, where he receives a one- to three-year sentence in state prison. This plan does not fool Evans, however, who plants undercover agent Hank Fallon (Edmond O'Brien) in Cody's cell in the Illinois State Penitentiary, where Hank goes by the name Vic Pardo. His main task is to find the "Trader," a fence who launders stolen money for Cody.
On the outside, "Big Ed" Somers (Steve Cochran), Cody's ambitious right-hand man, takes over leadership of the gang and the treacherous Verna throws in with him, feeling assured that Cody will never make it out of prison because Big Ed has paid an associate of his serving time in the prison, Roy Parker (Paul Guilfoyle), to kill Cody. In the prison workplace Parker arranges to drop a heavy piece of machinery on Cody but Hank pushes him out of the way, saving his life. Ma visits and vows to take care of Big Ed, despite Cody's frantic attempts to dissuade her. He starts worrying and decides to break out. Before he can, Cody learns that Ma is dead and goes berserk in the mess hall, slugging guards before being overpowered and dragged away to the infirmary. Although feigning a psychosis (loss of contact with reality), he concocts a plan to escape the prison. In the infirmary he is diagnosed as having homicidal psychosis and is recommended for a transfer to an asylum.
Cody takes hostages and escapes, along with his cell mates, including Hank, and also takes Parker along with him, planning for a "payback" for Parker's attempt to kill him. Parker is locked in the trunk of the getaway car. Later, when he complains, "It's stuffy, I need some air," Codyâblithely snacking on a chicken legâreplies, "Oh, stuffy, huh? I'll give ya a little air" and fires his gun several times into the trunk. The remaining men head for California. On hearing of Cody's escape, Big Ed nervously waits for him to show up. Verna tries slipping away but Cody catches her. Although it turns out that she was actually the one who murdered Ma by shooting her in the back, she convinces Cody that Big Ed killed Ma, and he guns down Big Ed. The gang welcomes the escapees, including Hank, for whom Cody has developed a genuine liking. Cody insists on sharing the proceeds from their robberies with him, stating, "I split even with Ma, didn't I?"
A stranger (Fred Clark) shows up at the gang's isolated country hideout, asking to use the phone. Everyone expects the stranger to be murdered ("Looks like Big Ed's gonna have company"). To Hank's surprise, a trusting Cody introduces him to the stranger, who is Daniel "The Trader" Winston, the fence whom Hank was to track down. Cody intends to steal the payroll at a chemical plant in Long Beach, California, by using a large, and empty, tanker truck as a Trojan Horse. Hank manages to get a message to Evans and an ambush is laid. The gang gets into the plant and makes their way to the payroll office, but as they begin to cut through the safe, the tanker's driver, "Bo" Creel (Ian MacDonald), recognizes Fallon ("He pinched me four years ago").
The police surround the building and Evans calls on Cody to surrender, but he decides to fight it out. When the police fire tear gas into the office, Hank manages to escape. In the ensuing gun battles the police kill most of Cody's gang, and he shoots down one of his men who tries to surrender. The police arrest Verna, who was parked in a getaway car across from the plant, and she tries to barter with Evans for leniency, saying she can convince Cody to surrender and then "you can do what you want with him", but Evans turns down her offer and she is taken away, hurling insults. Cody then flees to the top of a gigantic, globe-shaped gas storage tank. When Hank, a marksman, shoots Cody several times with a rifle, Cody starts firing at the tank and shouts, "Made it, Ma! Top of the world!" The tank and several adjoining ones explode, killing Cody.

Question: Why does Cody turn himself in?
Answer: present a false alibi