One night in Tampa, Florida, Mickey Duka leads the young Bobby Saint to an arms deal. The deal is brokered by a man named Arnold Creed, who speaks with an accent due to his knowledge of six languages. The exchange goes bad when FBI suddenly burst into the scene, and agitated, Arnold pulls out a gun. The police fire back, killing everyone except the cowardly Mickey, who is terrified when he sees Bobby's body.

The bodies are taken away, but Arnold Creed stands up unharmed, and speaks with an unaccented voice that he hates the undercover jobs he's always been taking. Arnold Creed is really Frank Castle, a FBI agent who is just retiring. This case had been his last, and he was upset at the death of the young man. The police are ill at ease when they discover who the young man is, Robert Saint, son of crime lord Howard Saint. When the news reach the Saint family, the second son Johnny orders Mickey to be bailed out.

Mickey is taken by the Saints to a club and beaten. He explains to Quentin Glass that Robert "Bobby" Saint came of his free will, and that the Arnold Creed, the man responsible for the deal, is dead. Howard is still unsatisfied, and kills another man who was supposed to be responsible for his son in front of Mickey. Meanwhile, Frank returns to his family, his lovely wife Maria and young son. Frank knows he has been neglecting them and forcing them to move constantly, but he promises London would be the last place to move to.

Before that though, they will go to a family reunion in Puerto Rico. Howard Saint goes to confirm his son's body, and when the police proved unwilling to share information with him, he orders his own men to find out everything about Arnold Creed. At the funeral of his son, Quentin informs Howard that Arnold Creed was a fake, that his real name is Frank Castle. Howard orders him killed, but his wife Livia adds that the whole family must also die. Howard agrees, while Johnny decides to go to Puerto Rico with the other assassins.

The assassins arrive just in time for another family outdoor beach party. Frank has gone inside with his father to look at some modified guns, they hear the gunshots. Though they fire back, they are unable to rescue anyone. The assassins completely slaughter the family, and although Maria and her son manage to escape in an SUV and drive away, the assassins eventually catch up and run them over on a pier. Frank, distracted by a knife fight, arrives too late.

The assassins return, and Johnny beats and shoots Frank himself, telling him "My father and mother send their regards". Quentin then torches the place, and they leave Frank for dead, not knowing that the explosion actually blasted Frank into the water. They return to the Saints & Sinners club, where they toast to "a score settled", and Howard gives Livia a pair of diamond earrings to celebrate. Quentin slow dances with Livia as Howard watches, then Livia strips off her dress for Howard when they are alone.

Candelaria, one of the Castle family's friends, rescues Frank. After he recovers, Candelaria takes him back to the Castle house. Frank picks up the guns his father had been showing him, as well as a T-shirt with a skull logo that his son had given him. Filled with anger, he is determined to have revenge and returns to America. He moves to a poor run down apartment building, where he starts modifying guns, the apartment he lives in, and his car.

Frank's actions catch the interest of his friends, the opera- and food-loving fat Bumpo, the piercing-covered skinny video gamer Spacker Dave, and the lovely Joan, who works as a waitress and is friends with Bumpo and Dave. One evening, Frank drives off in his car. He kidnaps Mickey and using a blowtorch, steak and popsicle, tricks him to tell all about the Saints. Mickey starts screaming (which badly scares Frank's neighbors).

Mickey eventually gives in, telling him that Howard Saint relies on two Cubans, the Toro brothers who control all gambling and prostitution up and down the Gulf coast. They give the money to Howard, who transports it to his bank in the Grand Caymans and "washes it". Frank releases Mickey, who discovers he was never really hurt, and freely talks about the Saints - whom he claims he hates; Mickey has been forced into their servitude.

With the information about the family's schedule and their closest friend Quentin Glass, Frank starts following them and taking photos. He learns about Howard's golf schedule, Livia's movie nights, Johnny's women and cars lifestyle and Quentin's secret homosexual trysts. Afterward, he drags his own gravestone out of the graveyard and plants it in Howard's golfing spot with the date of death removed.

Frank meanwhile confronts his former police colleagues in front of several reporters, asking why nobody was arrested for his family's death after five months. Learning that the police were too afraid of taking the Saints head-on, Frank takes matters into his own hands. He sneaks into the Saints' bank building, gets past two guards and gets to the vault. He hits one man unconscious and orders two others counting cash to fill up a suitcase with money. The rest of it he throws out the window, which draws in a noisy crowd trying to catch the money.

Frank, dressed in a long black coat and the skull t-shirt, confronts two of Saint's armed guards and kills them, he escapes easily into the crowd. The Tampa TV news reports the bank incident and identifies Frank. Bumpo and Dave look up Frank's history on the Internet and find his resume. Meanwhile, Howard is furious at the news, and receives a warning from the Toro brothers that if they lose more money they will be switching bankers.

Back in Frank's apartment, an angry former boyfriend bangs on Joan's door demanding to be let in. Bumpo and Spacker are scared, but Frank easily disarms the man and sends him away. Joan thanks him and says they are sorry for what happened to his family. Setting his revenge in motion, Frank breaks into Livia's car and calls Quentin on her phone with a device to disguise his voice. He blackmails Quentin Glass for $5000, saying he has photos "Mr. Saint would not want to see", telling Glass to come to a hotel.