Memento

The film opens with a sequence, played in reverse, where a man fatally shoots another in the head and takes a Polaroid photo of the dead man. The film continues as a non-linear narrative presented as an alternating series of black-and-white and color scenes.
The black-and-white scenes begin with former insurance investigator Leonard Shelby in a motel room speaking to an unknown caller. As a result of an attack by two men, Leonard has anterograde amnesia and is unable to store recent memories. He explains that he killed the attacker who raped and strangled his wife Catherine, but a second clubbed him and escaped. The police did not accept that there was a second attacker, but Leonard believes the attacker's name is "John G" or "James G". Leonard investigates using an elaborate system of notes, Polaroid photos, and tattoos to keep track of the information he discovers.
Leonard recalls Sammy Jankis, another anterograde amnesiac, from his insurance industry days. After tests demonstrated Sammy's inability to learn tasks through repetition, Leonard believed that his condition was at best psychological and denied his insurance claim. Sammy's distraught wife repeatedly asked Sammy to administer her insulin shots for her diabetes, hoping he would remember having given her a shot. However, Sammy administered each injection, giving her a fatal overdose.
The color scenes are presented in reverse order. In the story's chronology, Leonard self-directively gets a tattoo of John G's license plate number. Finding a note in his clothes, he meets Natalie, a bartender who initially mistakes him for her boyfriend Jimmy Grantz, whom she expects to meet. After offering to help Leonard, she provokes him into assaulting her and exploits his condition by blaming her bruises on Jimmy's associate Dodd. She explains that Dodd has been aggressively pestering her for money that Jimmy took to a meeting he never returned from. Leonard volunteers to confront Dodd and Natalie provides him with Dodd's address.
Dodd finds Leonard driving and questions him about his car, but Leonard escapes after Dodd attempts to shoot him. Leonard ambushes Dodd in the latter's hotel room and calls his contact Teddy, who helps drive Dodd out of town at gunpoint. Teddy advises Leonard not to trust Natalie, but Leonard finds a note warning himself not to trust Teddy. Feeling pity for Leonard, Natalie returns his favor by running John G's license plate through a database and provides him with the driver's license for a John Edward Gammell, Teddy's full name. Confirming his information on "John G" and his warnings, Leonard drives Teddy to an abandoned warehouse, leading to the opening sequence where he fatally shoots him.
In the final black-and-white scene, prompted by the caller, Leonard meets with Teddy, an undercover cop who has found "John G", Jimmy, and directs Leonard to the abandoned warehouse. When Jimmy arrives, Leonard strangles him and takes a Polaroid photo of the body. As the photo develops, the black-and-white transitions to the final (chronologically earliest) color scene. Leonard swaps clothes with Jimmy before hearing him whisper "Sammy".
Suddenly doubting Jimmy's role in his wife's murder, Leonard confronts Teddy when the latter arrives. Teddy explains that he helped Leonard kill the real attacker a year ago, which Leonard has no memory of. Since then, Teddy has cyclically exploited Leonard's condition and search for "John G" to bust drug deals and profit off them. Teddy reveals that Leonard's wife survived the assault and was diabetic. It was actually Leonard who overdosed his wife, and he had repressed the memory into Sammy's story to escape his guilt.
After hearing Teddy confess, Leonard burns the photograph of the dead Jimmy and the photo Teddy handed him that documented his successful revenge from a year ago. In a monologue, Leonard explains that he is willing to lie to himself to be happy and believe that his actions are meaningful. He targets Teddy, who revealed that he too has a "John G" name, by ordering a tattoo of Teddy's license plate number and writing a note that Teddy cannot be trusted. Leonard drives off in Jimmy's car to a tattoo parlor.