The film opens with Bobby Saint and Mickey Duka meet with their contact, European arms dealer Otto Krieg, during an arms deal at the Tampa, Florida seaport . The deal soon goes south as the FBI raid the deal and both Bobby and Otto are killed in the ensuing attack. Mickey is arrested on the spot but it is soon revealed that Otto Krieg was an undercover FBI agent named, Frank Castle. A former military and war vet. The sting operation was Castle's final assignment and decides to take an early retirement from the field and decides to celebrate at his father's resort in Puerto Rico. It is soon revealed at the funeral of Bobby Saint that is father is a well known crime lord, Howard Saint. Howard and his right-hand man, Quentin Glass, bribe the FBI for confidential information about Otto Krieg and find his true identity. At Bobby's funeral, Howard learns Frank is in Puerto Rico before leaving for London afterwards.. Howard orders Frank murdered, but his conniving wife, Livia, demands that they kill Frank's entire family as well to "settle the score".
in Puerto Rico, Saint's men, along with Bobby's twin brother, Tommy attack Castle's family and kill everyone at the home and Frank is shot and blown off the pier and left to drown in the ocean by Glass. He however, washes up on shore alive and finds a black shirt that his son was making him, which now has the form of a skull on the front of the shirt. Later, o nce his injuries are fully healed, Frank returns to Tampa and takes refuge in an old tenement building where three young outcasts named, Dave, Bumpo and Joan live. Frank kidnaps Mickey Duka and at first frightens him into giving him information about the Saints, but Mickey, who hates the Saints, willingly joins Frank as a mole. Frank begins his mission of revenge after learning that the police and the FBI have been bribed by the Saints to close the investigation of his family's murder. In the process, Frank sabotages Saint's money-laundering business and severs Saint's partnership with the Cuban mobsters, the Toro Brothers, and starts tailing Livia and Quentin and learns that Quentin is gay and has been carrying on a secret affair unknown to everyone but Livia.
Frank begins to set up photos and other types of clues to make it look like Quentin is having an affair with Livia, which starts to succeed. However, Tommy discovers that Castle is in fact alive and starts sending his various agents to kill him for good. He first sends a guitar player named Harry Heck who, after a brief car chase, is killed when Frank shoots a knife into Heck's throat. The second is the Russian, a behemoth who mercilessly beats Castle in a brutal fight, but is defeated when Frank throws boiling oil in his face, which blinds him long enough for Frank to throw him down the staircase, breaking his neck. Quentin and his men eventually seek out Castle themselves and attack Frank's friends going as far as pulling out all of Dave's piercings before Castle eventually kills them all off. Howard, unaware that Quentin is gay, and believing Livia to be capable of cheating on him, kills them both personally, neither of them knowing why.
Frank then attacks Howard's headquarters, the downtown Tampa nightclub, "Saints and Sinners", killing Howard's henchmen. He then finds John, whom he leaves to die by trapping him with an eight-pound, trip-wire activated grenade placed in his outstretched hand. Eventually, John's strength gives way and the grenade explodes, killing him as Howard hears his son crying for help. Frank wounds Howard with a pistol when Howard attempts to shoot him in the parking lot of the nightclub. Howard is then tied to a slow-moving car in the club's parking lot, but not before he reveals to Howard that Quentin and Livia were not having an affair and that he had set everyone up to see his family in both aspects die around him the same way Saint did. As Howard cries out over being tricked by Castle, several hidden bombs go off, and the destruction and wreckage culminates in the shape of Frank's iconic skull.
Not long afterwards, Frank prepares to commit suicide but after seeing a vision of his wife and son decides not to. He leaves a countless amount of cash left over from the Saint's estate to Dave, Bumpo and Joan before leaving. The final scene is Frank Castle looking over the ocean saying that he will punish crime and that Frank Castle is gone, call him: "The Punisher"