Full story
The film opens in 1996 at an orphanage. Young Cletus Kasady watches Frances Barrison, the girl he loves, get hauled off to the Ravencroft Institute. She has a sonic scream that can floor anyone. On the way there she attacks officer Patrick Mulligan, who shoots her in the eye. Mulligan thinks he killed her. Frances survives and spends years locked up with her powers suppressed.
In the present, Cletus is a serial killer on death row who will only talk to reporter Eddie Brock. During the visit, Venom studies the drawings on the cell wall and figures out where Cletus buried his victims. The story saves Eddie's career, but life with the symbiote is a mess. Eddie bans Venom from eating people, even bad guys, and the alien is always hungry for brains, cranky and wrecking the apartment. On top of that, Anne Weying tells Eddie she's marrying Dr. Dan Lewis.
On another visit, Cletus taunts Eddie, Venom lashes out and the killer bites Eddie's hand, swallowing a piece of the symbiote. Soon after, Eddie and Venom have a nasty fight and split up. Venom hops from body to body around the city and lands at a costume party, where he announces he's out of the closet. At Cletus's execution, the lethal injection fails: out of him bursts Carnage, a red symbiote that's far more violent. The two slaughter their way through the prison and strike a deal. Carnage helps free Frances, now called Shriek, and Cletus helps kill Eddie and Venom.
Cletus breaks Frances out of Ravencroft, burns down the old orphanage and decides to marry her. Mulligan arrests Eddie as a suspect. Anne, acting as his lawyer, tracks down Venom and talks him into making up with Eddie. Back together, the two escape from the police. Meanwhile, Cletus and Frances kidnap Anne and Mulligan and take them to Grace Cathedral for a wedding with an audience.
At the cathedral, Carnage has the upper hand. Venom gets an idea: he goads Frances into screaming, since sound pulls symbiotes off their hosts. With Cletus exposed, Venom devours Carnage and bites the killer's head off. The bell comes down on Frances. Mulligan survives, but his eyes glow blue, a sign that something strange is inside him. Eddie and Venom lie low on a beach in Mexico. In the post-credits scene, a blinding light drops them into a different hotel room, where the TV is talking about someone named Peter Parker. That comes back later.