When a man insults Richard Kuklinski Michael Shannon's girlfriend during a game of pool, he follows the man to his car and murders him by slashing his throat.
Kuklinski marries Deborah Winona Ryder in 1964, and the couple have two daughters.
On the surface, he presents himself as a normal and successful working man, but he harbors a dark, violent side that he keeps hidden from family and colleagues.
As a boy he and his younger brother endured brutal beatings from their immigrant Polish father, shaping them into emotionally disturbed, sadistic young men. His brother is serving a life sentence in prison for raping and murdering a 12-year-old girl.
Kuklinski works by dubbing pornographic films, but tells his wife he dubs cartoons. The mob-backed company he works for is shut down by Roy DeMeo Ray Liotta, a powerful New York City gangster. DeMeo tests Kuklinski by having him kill a homeless man, then hires him as an enforcer and contract killer.
In 1975, Kuklinski moves his family into a nice suburban home and starts telling people he works at a currency trading company. He is ordered to kill a man who betrayed DeMeo, but finds a teenage girl in the closet afterwards. Freezy, a freelance hitman also hired by DeMeo, almost kills her, but is stopped by Kuklinski.
DeMeo, already involved in a dispute with the Cali Cartel, is furious when he learns what happened and decommissions Kuklinski indefinitely. Needing money, he makes a deal to carry out murders contracted to Freezy in return for half the bounties.
Kuklinski's home life is beginning to unravel. He starts becoming incapable of hiding his inner rage and shows bursts of anger towards his wife and strangers.
DeMeo's boss Leo Merks hires Freezy to kill one of DeMeo's associates, which Kuklinski does by poisoning him with cyanide. DeMeo finds out about the hits Kuklinski has been doing behind his back and threatens to kill his family if they ever cross paths again.
Kuklinski then goes to Leo Merks to collect the bounty money, but Leo is furious that the job was not untraceable. He refuses to pay anything, prompting Kuklinski to shoot him dead.
Kuklinski's daughter is hospitalized after a hit-and-run by DeMeo. Both Kuklinski and Freezy have to go on the run to escape DeMeo, but Freezy insists they kill each other's families to prevent them from talking to the police. When Freezy reveals that he already knows Kuklinski's address, Kuklinski kills him.
Following an undercover sting operation, Kuklinski is arrested in 1986. His wife and kids had never suspected him of being a killer. Kuklinski admits to having committed over 100 murders, both for personal reasons and for profit, in his 22-year career. After being sentenced to two life terms in prison, he never sees his wife and daughters again.
As the movie ends, Kuklinski's only regret is hurting his family through the crimes he committed. In 2006, he dies in a prison hospital, from a rare inflammatory disease; the film's closing intertitles indicate that foul play was suspected in his death as he was scheduled to testify at the trial of a Gambino family underboss.
