In 1978, Joseph Pistone, a determined young agent of the F.B.I., successfully infiltrates the dangerous realm of the mafia. His journey begins when Lefty Ruggiero, a small-time mobster, catches his eye. Lefty is eager to sell a diamond, which Joseph, going by the name Don the Jeweler, identifies as a fake. However, Joseph offers to assist him in settling his debts, compelling Lefty to hand over his car as a show of trust.
This act earns Joseph Lefty's respect, and he quickly becomes entrenched in the mafia world, initiating a series of reports and photographs that provide the police with invaluable insights from within. Lefty, adept at spending money on losing bets, sees in Joseph the son he wishes he had, especially as he struggles with a non-producing son who is ensnared in drug addiction.
What Lefty is unaware of is Joseph's own family, largely forgotten amidst the chaos of his undercover life; he even misses his daughter's communion, highlighting the personal sacrifices he must endure. As Joseph's influence grows and the police's understanding of mafia operations improves, Lefty's status is bolstered by Sonny Black, a powerful figure in the Brooklyn mafia. Sonny, appreciating Lefty's efforts during his incarceration, commands respect but remains oblivious to Joseph's true identity.
As tensions mount, the FBI instructs Joseph to guide Sonny’s focus toward Florida, where it appears the mafia's operations remain largely untapped. Intrigued, Sonny decides to purchase a lavish casino and club, unaware that during its grand opening, the police will stage a raid, decimating their newfound venture and leading Lefty to suspect an informant within.
Unbeknownst to them, it is Sonny Red, a rival mafia boss, who tipped off the authorities, leaving Sonny Black vulnerable as he faces the prospects of retribution and dangerous power plays. With his rival eliminated, Sonny Black asserts his dominance within the organization, leaving Joseph caught in a quandary of morals and loyalties. He wrestles with the consequences of his undercover role, especially as it becomes increasingly clear that the first casualty might be Lefty, who, despite his flaws, has become a friend.
As Joseph grapples with his dual life, his marriage crumbles under the weight of his secretive work, prompting his wife to seek a divorce. Amid his internal conflict, he resolves to protect Lefty at all costs. However, when Sonny orders Joseph to execute Sonny Red's son, Joseph's cover is nearly blown by the timely arrival of the FBI, who pull him back from a life-altering decision.
In a devastating twist, the FBI ultimately exposes Joseph's true identity to both Lefty and Sonny, sealing their fates in a deadly conclusion. With a $500,000 bounty placed on his head by the mafia, Joseph must assume a new identity, realizing that while he may have regained his family, the scars of his undercover life will haunt him forever.
