Wade Wilson, a former special forces operative working as a mercenary, meets an escort named Vanessa at a local bar, and their one-night stand becomes a relationship. A year later, Wilson proposes to her, but is then diagnosed with liver, lung, prostate and brain cancer. Despite Vanessa's love, he detests the thought of her watching him waste away, and leaves her in the middle of the night.
A recruiter from a covert organization approaches Wilson and promises him abilities that will cure his cancer. Wilson reluctantly agrees. He is taken to a remote laboratory where he meets Ajax and Angel Dust, and instantly resents them. Ajax injects a mutation-activating serum into Wilson and subjects him to daily torture to activate it. When Wilson's body fails to respond, Ajax asphyxiates him in an oxygen chamber, developing an accelerated healing factor that cures him but leaves him disfigured. Wilson finds a way to destroy the lab and escapes his confines. He battles Ajax, but relents when Ajax promises a cure. Ajax then impales him with a rebar and leaves him for dead.
Wilson survives and attempts to return to Vanessa, but is afraid of her reaction to his disfigurement. After consulting his best friend Weasel, Wilson begins the task of tracking down Ajax to get the cure. He becomes a masked vigilante, adopts the alter ego "Deadpool", and resides with an elderly blind woman, Al. Following a string of leads, Wilson tracks Ajax to a convoy on an expressway. He executes the numerous convoy guards, subdues Ajax, and demands the cure. He is interrupted by Colossus and Negasonic Teenage Warhead. They try to convince him to join the X-Men, and Ajax escapes. They attach a restraint to Wilson, but he frees himself by severing his own hand, which regenerates overnight.
Ajax and Angel Dust go to Weasel's bar and learn about Vanessa. Weasel warns Wilson that she is in danger, and they go to the strip club where she works to warn her, but Wade hesitates. Ajax and Angel Dust abduct Vanessa and tell Wilson to meet them at a decommissioned helicarrier in a scrapyard. Wilson persuades Colossus and Negasonic to help him rescue Vanessa. They travel to the scrapyard and battle Ajax, Angel Dust and their team of mercenaries. As Colossus and Negasonic fight Angel Dust, Wilson kills most of the mercenaries and engages Ajax in a hand-to-hand combat atop the helicarrier. Negasonic accidentally destroys the equipment stabilizing the helicarrier, tipping it and scattering Ajax and Wilson. As Colossus carries Negasonic and Angel to safety, Wilson saves Vanessa and injures Ajax. When Wilson demands his cure, Ajax laughs and reveals there is none. Wilson kills Ajax, despite Colossus's pleas to spare him. Vanessa is angry with Wilson for abandoning her, but they reconcile and kiss after she learns why he did not return sooner.