Some background, courtesy of Kerry Wilkinson:
Back during the Secret Wars, Spider-Man got himself what he thought was a new costume. It turned out to be an alien symbiote that wanted to possess his body. Spidey rejected it, however and it was left holed up in a church tower.
Shortly afterwards, Eddie Brock, a journalist with the Daily Globe lost his job after his Sin-Eater identity story was proved to be fake by Spider-Man. Wanting to end it all, he first went via a church to say his last goodbyes. His hatred of Spider-Man was picked up by the symbiote however and the two bonded to form Venom - a crazed killer who wanted nothing more than to kill Spider-Man.
In their many battles, Spider-Man triumphed consistently - even though his Spider-Sense doesn't pick up on Venom's presence. One time when Brock was in prison, however, the symbiote gave birth to another version of itself. This new symbiote bonded with Cletus Kasady - Brock's psychotic cellmate - and together the two became another crazed killer, Carnage.
When Carnage first escaped, Spidey teamed up with Venom and the Fantastic Four to save the city and re-incarcerate Kasady. Since then, Venom has re-escaped too and, after Spidey helped save Brock's ex-wife, formed an understanding with Spider-Man that he won't cause anymore trouble and that the two of them will stay out of each other's way. Most recently of all, in one of the best Spider-Man books of all - Spectacular 200 - JM DeMatteis scripted the death of Harry Osborn. He had gone crazy as the Green Goblin, wanting to blow up most of the city's leaders. After MJ and little Norman entered the building he was fighting Spider-Man in, however, he came to his senses, even saving Peter from the explosion, before he died in front of them.
Carnage and his cronies comandeer an empty warehouse for their headquarters, and the first guest is Demogoblin. Meanwhile, at the Parker home, Peter and Mary Jane argue about what to do with the prone form of Venom lying in their doorway. After Mary Jane storms out, Peter helps Venom onto the couch, and is flabbergasted that the villain wants to seek an alliance with him. Spider-Man knows that he can't tackle Carnage and his team alone, and so goes off to seek advice from Felicia. Instead of advice, however, she offers assistance in the form of the Black Cat.
Meanwhile, Mary Jane arrives at the home of Aunt May, seeking comfort. At Carnage's headquarters, though, the Demogoblin has arrived seeking justice. In the end, however, he simply agrees to join Carnage's team. Spider-Man and the Black Cat have also agreed to team-up with Venom. Back at Carnage's headquarters, Cloak has arrived to exact justice on the one who killed his beloved Dagger. He does not fare well, but one of Shriek's attacks alert Spider-Man and his allies to their location.
The two groups merge, and once again prove to be too evenly matched. The battle ends when Carnage's team brings down the now-burning roof upon the heroes. As Carnage and his comrades flee, the Black Cat is trapped under the roof and Venom is weakened by the flames. Spider-Man is faced with a decision: save Black Cat, or go after Carnage.
The crossover is finally beginning to shape-up, though it seems to be an early experiment in decompressed storytelling. You could have easily fit this issue and the previous entry of the crossover into one book. This second battle between the two teams is slightly more believable than the first, especially with the additions of Venom and Demogoblin to the mix.
Art-wise, the book was strong. My only question is how soon does this take place after "Amazing Spider-Man #378?" The scene where Venom entered the Parker's apartment clearly took place at night, with Mary Jane and Peter in their "pajamas". In this issue, Mary Jane is wearing a purple dress and Peter is in a pair of jeans, and it's daytime.
One continuity snafu brings the book down, not to mention the fact that very little actually happens.