rated PG-13

There are at least three different movies jammed into “Spider-Man 3.” Two of those movies are all-out superhero slugfests that balance all the punches and explosions with some decent acting and compelling, character-driven subplots. The third movie is some sort of misguided musical comedy, complete with silly montage sequences and a cringe-worthy dance number. The changes happen so quickly that viewers will find themselves confused and looking around, wondering if, perhaps, someone switched the movie without telling them.

Taken as a whole, “Spider-Man 3” doesn’t make a damn bit of sense. The first hour or so of the film is everything you’d expect from a Spider-Man movie: sharp special effects, exciting fight sequences and a deft blending of believable characters with everyday worries. The story picks up where “Spider-Man 2” left off: Peter Parker (Tobey McGuire) continues to juggle his duties as New York’s friendly neighborhood web-slinger with school and his girlfriend, Mary Jane Watson (Kirsten Dunst), to whom he plans to propose. On top of that, Peter struggles to repair a damaged relationship with friend Harry Osborne (James Franco), son of the Green Goblin, the nefarious villain who died in combat with Spider-Man way back in the first film. Harry blames Peter for his father’s death and soon picks up his father’s super-villain identity in order to terrorize Spider-Man. Meanwhile, prison escapee Flint Marko (Thomas Hayden Church) longs to return home to care for his sickly daughter, but instead gets chased into the middle of some bizarre science experiment and is transformed into a living pile of sand. Later, Spidey learns that Marko, now calling himself “The Sandman,” might be the man who killed his uncle.

And that’s just the first half of the movie. A second plotline involves alien goo that turns Spidey’s costume black and makes him slightly evil. That’s where things start to go wrong. Up until this point, “Spider-Man 3” rocked. A thrilling, disorienting and emotional fight sequence between Spider-Man and the Green Goblin’s son in the first half hour catches viewers by surprise. Peter and Mary Jane’s various relationship troubles, along with Peter’s nervousness about proposing and his attempts to patch things up with Harry (after the giant fight, that is), could have easily carried the whole movie without all the evil Spidey nonsense that followed.

Taken over by the black suit, Peter Parker isn’t so much evil as just a brooding jerk. He wears his hair down over his eyes, struts down the street and shoots flirtatious glances with the same ease with which he shoots out webs. Spidey becomes totally emo and slightly sleazy.

Spidey’s dark side further manifests itself when, after Mary Jane dumps him, he starts dating his classmate Gwen Stacey (Bryce Dallas Howard). For their first date, Peter takes Gwen to the jazz club where Mary Jane works as a singing waitress. He goes on to ruin the evening for both women (and the audience) by staging an elaborate, embarrassing dance sequence. Not only is this Spider-Man a jerk, but he’s also a bad dancer.

Spidey soon realizes that the goo that makes up the black suit is some sort of alien parasite, one that amplifies feelings of anger and aggression in its host. He finally gets rid of the alien goo, only to have it discover a new host: Eddie Brock (Topher Grace), a disgraced photographer who blames his career troubles on Peter Parker. For reasons cobbled together for the sake of facilitating a last-minute ending, Brock, now known as Venom, teams up with Sandman in order to kill Spidey. The film’s climax is decent enough, but like the rest of the movie, it feels bloated and is over too soon.

There are plenty of good things about “Spider-Man 3,” though after Peter Parker’s goofy descent into darkness and the rushed ending, you’re liable to forget all the good stuff. Both Dunst and Franco give solid performances, and while McGuire seems less confident and whinier than he did in previous Spider-Man movies, he’s still likeable. Director Sam Raimi provides his usual visual flair, with plenty of brutal action sequences and vertiginous action shots of Spidey swinging between massive skycrapers. And there are some touches of comedy that work—Bruce Campbell has a cameo as a French maitre’d who comes to Peter Parker’s aid at a very important moment. It’s a funny, slightly awkward, light-hearted scene that perfectly captures what makes Spider-Man such a great hero: he’s a regular person first and a superhero second. It’s the nimbleness with which Spider-Man treads the line between light-hearted and dark, between hero and everyday man, that made the first two Spider-Man films so watchable.

But it’s hard for a movie to be nimble when it’s stuffed with too much plot and weighted down with too much darkness. The easy-swinging Spider-Man found in previous entries can still be spotted in “Spider-Man 3,” but the glimpses are fleeting and remind you only of how badly the rest of the film is fumbled. Raimi and the rest of the cast make it easy to get caught in Spider-Man’s web, but by the time the film ends, most viewers will be searching for a way out.