The third part of a movie trilogy is always a tough nut to crack, and the pressure has driven many directors insane. “Superman 3” featured a drunk Superman battling Richard Pryor; “Return of the Jedi” was infested with Ewoks singing a musical number entitled “Yub Nub”; and while “Spider-Man 3” doesn’t sink to those levels, the second edition of the franchise was such an improvement on the first that this installment feels like a giant step backward.

The first hour of the film is deceptively good. Tobey Maguire’s Peter Parker is still the world’s perkiest grad student, sporting Frodo-sized blue eyes that make him the object of desire for aspiring actress Mary Jane Watson (Kirsten Dunst), who has gone from starring in a revival of “The Importance of Being Earnest” (in “Spider-Man 2”) to starring in one of those deliciously tacky Broadway musicals that only appear in movies. The fly in the ointment is Peter’s best friend, Harry Osborn (James Franco), who wants to kill Peter because he knows that he’s Spider-Man and he thinks, erroneously, that Spider-Man killed his father, the Green Goblin.

At this point, the plotlines multiply like drunken bunnies. Harry takes up his dad’s Green Goblin gear and vows to smash Peter like a piñata; an escaped convict, Flint Marko (Thomas Haden Church), gains the ability to transform himself into kitty litter and he uses his super-absorbent, odor-free powers to go on a crime spree to raise money for his daughter’s operation; hungry newcomer Eddie Brock (Topher Grace) tries to ace Peter out of his photographer’s job at the Daily Bugle; Gwen Stacy (Bryce Dallas Howard) develops a crush on Peter; bad reviews bring down Mary Jane’s career; and a black and goopy alien slithers off a crashed meteor and into Peter’s life, obligingly forming itself into a spiffy black spider-suit. Then the kicker: It seems that Flint Marko was the miscreant who snuffed Peter’s beloved Uncle Ben!

Two action scenes early in the film top anything you might have seen in the other two installments. In one, a crane unwisely perched on top of a skyscraper runs amok … during a fashion shoot! Models in peril! In the other, Harry Osborn ambushes Spider-Man in what may be the only 3-foot wide, 2-mile-long alley in New York City. Director Sam Raimi bounces his actors off of pipes, walls, and fire escapes like a jazz drummer blasting out a crazy solo, pushing the camera up to stomach-clenching heights and then letting it fall in nauseating swoops — proof that computer-generated pixels can get your blood pumping if they’re done right.

The “Spider-Man” movies are not careful Caravaggio studies of shadows, but big, bold Jasper Johns pop opera canvases, bursting with bright colors, graphic panache, and American flags. But by the time this film’s last subplot has lumbered off the runway, it feels as if the filmmakers have exhausted their sense of invention. From that point on the movie reeks of a joyless “Let’s wrap this up” predictability.

That chic black spider-suit brings Peter’s dark side to the surface and as he becomes consumed with thoughts of avenging the death of his uncle, he wears the suit more and more and it turns him into Evil Peter. Evil Peter combs his bangs down, struts around Manhattan shooting “groovy fingers” at the ladies, dances sexy, and wears a black suit with a black shirt and tie. “Who are you?” asks a perplexed Mary Jane. Eurotrash, apparently.

When Evil Peter plays jazz piano in a club and then does a sassy tap dance on top of the bar the entire audience will stop and wonder, What was that sound we just heard — that whoosh and that chomp? That was the sound of a shark being jumped. Character motivations go out the window, and the world of the movie becomes whatever the harried writers need it to be in order to bring this thing in under three hours. They even violate the cardinal rule of action films: The big finale pales in comparison with the movie’s earlier action scenes.

The new additions to the cast are terrific, but terrifically ill-used. Mr. Church is a great Flint Marko, a man who not only seems to be missing his upper lip but who speaks as if his throat is packed with sand. And Mr. Grace makes a meal out of the crumbs of character that he’s given. The returning cast is up to snuff, except for Ms. Dunst, who gives the performance of a woman who was out late the night before.

Sadly, this is a franchise that has run its course. “Spider-Man 3” isn’t a bad movie, but it’s not a very good one. The first “Spider-Man” was a novelty, the second was a high-flying surprise. But this third flick spends its first hour weaving a fascinating web only to spend its last hour getting all gummed up in it, and then hosing the entire, goopy mess down the drain in the last 20 minutes.