The best superhero series (“Superman,” “Spider-Man”) didn’t start plummeting until their third installments, but sequelitis hits Tony Stark pretty hard just the second time around. The first “Iron Man” was only middle-grade, but this follow-up is considerably less than that. Talky, curiously depressing for most of its running-time, weighed down by too many heroes and an army of uninteresting villains, and messily prosaic in the big action moments, “Iron Man 2” suggests that this franchise is already running on fumes.

As the script cobbled together by Justin Theroux begins, Stark (Robert Downey, Jr.) is a popular celebrity, extravagantly presenting himself as the keeper of world peace. But his manic attitude conceals a dark reality: the power source keeping him alive is poisoning his blood, and will soon reach fatal levels. (We learn later that he’s also depressed over the belief that his daddy—played in news footage by John Slattery—didn’t love him.)

Stark is also being pressured to turn over his Iron Man technology to the DOD by snarky Senator Stern (Garry Shandling, looking really bloated), who’s in league with rival weapons manufacturer Justin Hammer (Sam Rockwell), a sleazy fellow whose products may be junk but who’ll do anything for a buck and has a gift for self-promotion (as well as the support of Stern). Meanwhile, in Russia, Ivan Vanko (Mickey Rourke), whose father—an embittered former colleague of Howard Stark—dies, leaving his files to his son and thus allowing him to build (with what funds are unclear) his own battle suit featuring electrical “whips.” And in the guise of Whiplash he takes familial vengeance on the Starks by attacking Tony as he exhibits his risk-taking lifestyle by driving in a car race in Monaco.

The peregrinations of the plot from this point on become needlessly complicated, and frankly rather dull. Hammer joins forces with Vanko to produce a weaponized suit that will get him government contracts and reduce Stark Industries to also-ran position. Tony grows more dissolute, prompting his girlfriend/business partner Pepper Potts (Gwyneth Paltrow) to grow uncertain about his mental state and his buddy Rhodey (Don Cheadle) to don one of the Iron Man suits and bring him to his senses by force. And through the agency of Nick Fury (Samuel L. Jackson), head of the super-secret spy outfit called SHIELD, Stark is brought to a posthumous understanding with his father that provides an answer—a rather silly one, but this is after all comic-book stuff—to his problem of blood toxicity. (The excessive screen time given to Jackson, who merely preens and pontificates, and Scarlett Johansson as one of his operatives, who poses as a secretary in Stark’s office, is apparently dictated by the desire to promote The Avengers, as their group is known, in preparation for an upcoming movie centered on them.)

Everything culminates in a big final battle, of course, but in this case Iron Man isn’t alone. He partners with the suited Rhodey to take on Vanko and his army of droids, which rather resemble mini-Transformers, or perhaps a battalion of Robocops without the human face. And meanwhile Johansson’s leather-clad heroine, assisted (none too successfully, it must by noted) by Stark’s chauffeur (played by director Jon Favreau) confronts a bevy of security guards at Hammer’s headquarters and Hammer himself is dealt with by Pepper. This last reel is all over the place as the focus shuffles between the simultaneous fights, diluting the impact of all of them.

What energy “Iron Man 2” possesses comes not from the desultory script or the choppy effects, but the human performances. Once again Downey brings his mixture of smug cockiness and ultra-cool snideness to Stark, tossing off his dialogue with aplomb; a pity the lines aren’t better. His glumness in the more morose, self-destructive moments, on the other hand, pretty much brings matters to a halt. Still, his innate charm mostly compensates for the character’s growing obnoxiousness. Nobody else in the cast makes much of a positive impression—Paltrow is pallid and Cheadle dully supportive, and while Johansson looks sexy in her leather outfit, her unvarying facial expression isn’t entirely made up for by her striking figure, while Jackson’s stentorian delivery is something we’ve seen too often from him before. Rockwell, a good actor, is simply embarrassing as Hammer, prancing about like a bargain-basement James Bond bad-guy to diminishing effect. Then there’s Rourke, who seems to be constantly winking at the audience in a vain effort to convince us how much fun we’re having watching him stroll through a role in which his major effort at characterization comes from his scraggly hair and multiple tattoos. And the cameos by “journalists” Christine Amanpour, Bill O’Reilly and the inevitable Larry King fall flat.

On the technical side of things, the picture has one great point in its favor—it’s not in 3-D (although the advantage is almost cancelled out by John Debney’s bombastic score). Otherwise the production design (J. Michael Riva) and art direction (David Klassen) are colorful if garish, and Matthew Libatique’s cinematography makes even the night flying scenes bright. A pity the same adjective can’t be used to describe Theroux’s uninspired script, or “Iron Man 2” as a whole. The suit may still be shiny, but as a movie Iron Man already shows signs of needing RustOleum treatment.