This film should start out in an asylum a year after the last where Eddie Brock is thrown in across the hall from serial killer Cletus Kassidy (this will set up the events for later in the film). In the mean time, Peter and Mary Jane are in over their heads in wedding preparation. Spider-Man’s reputation still isn’t all that good, but Jameson has let up on the libel a little bit. The idea is that things should seem pretty calm and easygoing before things erupt. The symbiote has been kept in lockdown at a military lab in the city. All it takes is one curious scientist to open the chamber in which the symbiote is held and hell is unleashed. The symbiote takes over the scientist for a brief time as it goes out in search of Eddie Brock. The scientist breaks into the asylum and frees Brock. The symbiote quickly moves back to its former host, and Venom afterwards kills the scientist. Despite Brock’s hatred for Kassidy, the symbiote splits itself in two and the other half clings to the madman, spawning Carnage.
Kassidy’s adrenaline feeds the symbiote much better than does Brock, giving Cletus a great level of control over Carnage. Venom escapes from the asylum, whereas Carnage stays behind and kills everyone inside. Spider-Man and Venom soon come into conflict again. Brock’s lowering adrenaline makes for a weaker Venom and he is more easily defeated (that, and Spider-Man’s recent happiness has inspired confidence). Venom tries to flee, but Spider-Man pursues him. In the same instant that Spider-Man catches up with Venom, he is attacked by Carnage. Carnage almost kills Spider-Man, but Venom (being almost jealous that he doesn’t get the kill) throws Carnage off. Carnage spews some insults Venom’s way and the two part ways, leaving Spider-Man down and injured. Mary Jane urges Peter to give up Spider-Man, but Peter refuses (as long as Venom and Carnage are out there, Peter is a target whether he dons the suit or not). Brock begins taking injections of adrenaline to quell the symbiote’s hunger (this gives Brock more control over Venom, but not enough to matter). Carnage makes a statement by attacking Peter’s Aunt May, almost killing her. Spider-Man goes on a manhunt for both Venom and Carnage. The violence and mind games continue, causing Spider-Man, Venom, and Carnage’s hate for each other to grow. The climax comes with a three-way fight between them (should be the longest, most violent fight scene in all six films; if anyone takes one memory from the film franchise, it should be this fight).
The fight ends with Brock left unconscious, Kassidy separated from the symbiote and unconscious (Brock should be separated from his symbiote before the fight is over, causing a merger of both symbiotes), and Spider-Man mortally wounded. After this, Peter and Mary Jane will cancel their wedding, knowing that marriage is not an option in their relationship.
Dr. Connors will destroy the symbiote using a sonic wave chamber. The very end of the film should have Scorpion attacking Jameson on the city street and Spider-Man will swoop in just a second or two before the credits roll (no, this doesn’t indicate a sequel, but it shows that Spider-Man is no where near done).