Plot

The movie starts in 1996, where a young Cletus Kasady watches as the love of his life, Frances Barrison, who is able to manipulate sound through her screams, is taken away from St. Estes Home for Unwanted Children. On the ride to Ravencroft Institute, she uses her sonic scream powers to try and escape atticking a young police officer, Patrick Mulligan. Mulligan shoots Barrison in the eye as he suffers an injury to his ear from her scream. Mulligan believes he killed her, but Barrison is alive and taken to the facility, which is immune her powers.

In the present day Mulligan is now a detective, he contacts Eddie Brock to speak to Kasady, a serial killer, since he refuses to talk to anyone who isn't Brock following the interview they did together a year prior. After Brock's visit, Venom, his symbiote, uses his photographic memory to figure out where Kasady hide the bodies of his victims, this gives Brock a major boost in his career. Brock gets contacted by his ex-fiancée Anne Weying, she tells him that she is engaged to Dr. Dan Lewis, which displeases Venom a lot.

Kasady, is found guilty of his crimes and is sentenced to death by lethal injection. Kasady invites Brock to San Quentin State Prison, where he is detained on death row, to attend his execution. When Brock goes to get Kasady's story, Venom is provoked by insults towards Brock and tries attack Kasady. Kasady the bites Brock's hand and ingests a small part of the symbiote. Back home, Venom, wants more freedom and to eat bad people causing an argument with Brock. The two have a physical altercation leading to Venom sperating from Brock and them both going their own ways.

Kasady's execution fails when a red symbiote emerges, blocking the injection. The symbiote introduces himself as Carnage then goes on rampage through the prison that frees the inmates and kills the guards. Kasady and Carnage then make a deal: Carnage will help Kasady break out Barrison from Ravencroft, and Kasady will help him kill Brock and Venom. Mulligan calls Brock to warns him about the situation. In Ravencroft, Kasady and Carnage free Barrison, and they travel to St. Estes to burn it down.

Mulligan, although suspicious of Brock because of his interactions with Kasady before Carnage emerged, takes Brock to the police station for questioning. Brock won't answer Mulligan's questions and contacts Weying to be his lawyer. Brock tells her that Venom has separated from him but needs the symbiote to fight Carnage. As Venom makes his way partying through San Francisco by hopping from body to body, Weying finds and convinces him to forgive Brock. Weying merges with Venom and together they break Brock out of the police station. Brock and Venom make their amends and fuse together again.

Kasady takes Mulligan hostage as Barrison kidnaps Weying, failing to find Brock. Barrison tells Dan Lewis information on where Weying is, and Lewis gives it to Brock. Kasady and Barrison plan their married at a cathedral, when Venom appears to fight Carnage. Barrison then seemingly kills Mulligan by hanging him onto a chain. Venom holds his own against Carnage for a while but he is eventually overpowered. Carnage decides to kill Weying atop the cathedral. Venom rescues Weying in time and provokes Barrison to use her powers again since her screams causes both symbiotes to separate from their hosts all while cathedral is collapsing. The bell from the cathedral falls on top of Barrison killing her.

Venom saves Brock by merdging with him before the impact for the crumbling cathedral can kill him. Carnage tries to fuse with Kasady again, but Venom devours the symbiote before he can. Kasady states that all he wanted was to be Brock's friend, but Venom then bites Kasady’s head off. While Brock, Venom, Weying, and Lewis escape the cathedral, a still-living Mulligan's eyes flash blue. Brock and Venom then decide to take a vacation while they question their next steps. As Venom is telling Brock about how the symbiotes' know of other universes, there is a blinding light that transports them from their hotel room into another room, where they watch a news report from J. Jonah Jameson revealing Spider-Man's identity as Peter Parker on television.