Principal photography of the film took place in Boston between May and August Bulger lives with his longtime girlfriend, Lindsey Cyr, and their young son, Douglas. After the Angiulo Brothers send a motorcycle riding assassin who murders a Winter Hill soldier, Whitey becomes an informant for Connolly.
Connolly believes he can infiltrate the Angiulo Brothers' organization with Whitey's help. Although Whitey hates the idea of being a rat , he understands what the resultant protection would afford him, his gang, and his family. Douglas suffers from Reye syndrome , leading the devastated Lindsey to remove him from life support over Whitey's furious objections.
Whitey increasingly exploits his status as an informant, using Connolly's "protection" as a cover for his crimes. When Connolly demands information on the Angiulos' racketeering locations, Whitey gets pictures of the rival gang's hideouts, allowing the FBI to plant wiretaps.
Blinded by his past, Connolly grows closer to Whitey and the gang and even invites them to his house for a cookout. His wife, Marianne, sees negative changes in her husband as his agent-informant relationship with Whitey grows, including accepting expensive gifts and money from the gang. An associate, Brian Halloran, fears for his own life and goes to the FBI to report Whitey's involvement, much to Connolly's displeasure.
Connolly then tells Whitey of Halloran's accusation, resulting in the murders of Halloran and an unnamed accomplice. Following his mother's death, Whitey's behavior becomes increasingly violent and unpredictable, deteriorating his informant relationship. When "bulldog" prosecutor Fred Wyshak is appointed the new assistant U. Attorney in Boston, Connolly attempts to make friends and perhaps divert his attention from Whitey. Wyshak bluntly refuses and demands the FBI arrest him.
