The film is presented in flashbacks by a Briton named Wilson (Stamp).
Wilson travels to Los Angeles to investigate the death of his daughter, Jenny. She reportedly had died in a car accident, but Wilson suspects she was murdered. Recently released from a British prison, he is a hardened man. Arriving in Los Angeles, he meets Jenny's friends Eduardo (Guzmรกn) and Elaine (Warren) and questions them. Finding they pass his initial inquiry, he elicits their help in investigating Jenny's death. One suspect that emerges is Jenny's boyfriend, a record producer named Terry Valentine (Fonda). Investigating him it is learned that besides his legitimate record company business, Valentine has involvement in drug trafficking. His involvement is managed through his security consultant, Avery (Newman). Wilson locates a warehouse used by the drug trafficker, and questions the men there. Laughing at him, they beat him, insult his daughter and throw him out onto the street. Undeterred, Wilson draws a hidden pistol and returns to the warehouse, shooting dead all but one of the employees. As the survivor flees, Wilson shouts after him "Tell him... I'm coming!"
Back with Elaine and Eduardo, Wilson reminisces about his earlier life with his daughter, whom he remembers only as a child. Worried her father would be sent away to prison, she would threaten to call the police whenever she found evidence of the crimes he had been involved in. He recalls she never followed through on her threats, because she loved him and it became a sad joke between them. However, his life of crime put a strain on his family. He ended up in prison after the men he was involved with sold him out to the police.
Seeking more information from Valentine, Wilson and Eduardo sneak into a party held at Valentine's house. Once there Wilson searches for evidence of Valentines involvement. He finds and steals a picture of Jenny. Attracting suspicion from Avery, Wilson is accosted by a guard whom he swiftly head-butts and throws over a railing to his death. Wilson and Eduardo then flee, but are chased by Avery, who rams their car. Wilson rams Avery's car in return, forcing it over a cliff. He and Eduardo escape, but not before Avery hears Eduardo call out Wilson's name.
Avery hires a hit-man, Stacy (Katt), to track down and kill Wilson and Elaine. Avery is prevented from making the hit by agents of the DEA, who have been monitoring Valentine as part of their investigation of a Mexican drug lord. Wilson and Elaine are taken to meet a DEA investigator. The meeting makes it clear the DEA is after the drug lord who used Valentine to launder money, and the agents will not interfere with Wilson's personal mission. The head agent lets Wilson see their file on Valentine, including his home addresses. Meanwhile, Stacy and his partner, angry at their beating by the DEA agents, plot to double cross Avery and Valentine.
Avery moves Valentine to a safe house in Big Sur, but Wilson knows its address. That night the grounds are penetrated. Avery's guards shoot an intruder, but it turns out the man killed is Stacy. Avery and the guards then engage in a shootout with Stacy's partner, resulting in several deaths. Valentine flees to the beach, but Wilson is still in pursuit. Falling on rocks and breaking his ankle, Valentine cannot escape. Wilson walks up as Valentine begs for his life, admitting his involvement in what happened. He says Jenny had found out about his drug ties. Confronting him about it she picked up the telephone and threatened to call the police. During his desperate attempt to stop her, she fell and accidentally broke her neck. To deflect attention from Valentine over her death, Avery then staged a car accident as the cause.
Haunted by the tale, Wilson knows his own involvement in crime led Jenny to act as she did, repeating the same half-serious bluff they had shared so many times. She would never have turned Valentine in, either. He turns away from Valentine, allowing him to live. Wilson makes his farewells to Elaine and Eduardo, and returns to London.

Question: Agents of which group prevented the hit planned on Wilson and Elaine?
Answer: The DEA.
Alternative answer: DEA.