Nathan Caine, an introverted assistant manager with congenital insensitivity to pain (CIPA), works at a credit union in San Diego. Despite his condition, co-worker Sherry Margrave pursues a relationship with him. After meeting his middle school bully at a bar, Nathan and Sherry prank him with hot sauce and spend the night together.
On Christmas Eve, a gang led by Simon Greenly robs the bank and takes Sherry hostage. Nathan impulsively steals a cop car to pursue her, but follows the wrong car and ends up killing robber Ben Clark in a restaurant. Nathan burns his hand while retrieving Ben’s gun. He tracks Ben’s tattoo artist, hoping to find Sherry. At Ben’s house, Nathan is trapped in a snare. He calls his gamer friend Roscoe for help. Together, they overpower and kill Ben’s brother Andre. The police arrest Roscoe, who has switched clothes with Nathan.
Nathan locates Sherry and Simon at an auto shop. He learns Sherry is Simon’s sister, and she had feigned interest in him for the heist but has since fallen for him. Simon tries to kill Nathan, but he is shot by Coltraine while Sherry and Nathan are arrested. Simon then shoots Coltraine dead and escapes in an ambulance. Sherry frees herself from the handcuffs and pursues.
Nathan regains consciousness and breaks free from the handcuffs, defibrillates Simon, causing him to crash. Simon breaks Nathan's arm as Sherry arrives. Just as Simon tries to kill Sherry, Nathan injects himself with adrenaline and kills Simon by sticking his exposed arm bone through Simon's jaw. He finally falls unconscious, exhausted from his injuries.
Nathan wakes in the hospital two weeks later to find out he's been sentenced to 6 months of house arrest. One year later he visits Sherry, who is near the end of her sentence for her role in the robbery. They share cherry pie. The prison guard leads Sherry back to her cell, and Nathan smiles knowing they'll be together when she's released.