“A passable knock-off”: That’s how the man in the Iron Man mask, the obscenely rich but heartsick industrialist Tony Stark, played by Robert Downey Jr., characterizes the electro-weaponry wielded by his Slavic adversary (Mickey Rourke) in “Iron Man 2.”

Much of this scattershot sequel to the 2008 smash feels like a passable knock-off as well. Here and there, director Jon Favreau’s diversion takes us back to the considerable satisfactions of the first “Iron Man,” whether in action mode, such as a nifty vivisection of metallic villainous drones (provoking the sole round of applause at the screening I saw), or in a Downey Jr. wisecrack uncorked just so, usually in the form of a non-apology apology to long-suffering Pepper Potts (Gwyneth Paltrow, back like Downey Jr. from the first picture, though somewhat sidelined).

It’s a curious, off-center sequel in some ways. The screenwriter Justin Theroux — also an actor, and a good one; he scored as the arrogant director in David Lynch’s “Mulholland Drive” — can be very witty. At one point Stark’s rival weapons manufacturer, a storm cloud of smarm played by Sam Rockwell, delivers a sales pitch for his latest and greatest weapons of mass destruction. In most comic-book movies the scene would’ve been handled either as grim exposition or aggressive camp. Here, it’s a sprightly riff on that wonderful, awful gun dealer scene in “Taxi Driver.”

“Iron Man 2” has a harder time with matters of story clarity and momentum. It’s a crowded blockbuster indeed, with Stark searching for a cure for his failing heart (he’s up to 80 ounces of chlorophyll a day) while fending off Senate investigators who want control of the Iron Man peacekeeping arsenal.

Ivan Venko, alias Whiplash, has it in for Stark and Stark’s family, for vaguely defined reasons.

Engaged in a virtual slink-off with Paltrow, Scarlett Johansson’s a Stark Industries employee (“from legal”) with motivations of her own. Meantime Stark’s old pal Rhodey (Terrence Howard in the first one, Don Cheadle getting more to do in the sequel) has conflicting loyalties when it comes to the party boy, which leads to a fuzzily motivated smackdown at Stark’s oceanside pad.

A movie like this can handle a large character roster, but it helps if the story retains clean lines and a sense of propulsion. “Iron Man 2” sags and wanders in its midsection. And Stark himself is more louche this time, more narcissistic and less fun. What you miss are the little things, the relatively low-tech charms of Stark futzing around in his swank homemade lab as he created the first suit. In “2” Stark wrestles mainly with the grind and excesses of celebrity as an established superhero. (Moviegoers have been down that road a lot, in everything from “Spider-Man 3” to “Watchmen.”) Also, and I think this is key: The first “Iron Man” celebrated Downey Jr.’s Stark as a sardonic, charismatic winner while questioning, for a while, anyway, the means by which Stark’s family made its billions. The new film is far more in love with its hardware; some of it veers too close to “RoboCop” territory for the good of the Marvel Comics mythology.

What saves it is Favreau’s work with the cast. He’s still no great shakes as a shaper of action sequences, but at least he and Theroux realize there’s more to a zillion-dollar franchise item than setting up the next destruction showcase. It helps that nearly every performer on screen has a verifiable, useful sense of humor.

Downey Jr. and Rourke never quite emerge as epic contenders in a death match, but they’re terrific faces to watch in any circumstance.

There are times when the conversational dialogue flies past “loose” and lands in “chaotic,” with everybody nattering at once. It’s a good comic impulse, though.

I suspect Theroux is a talent best paired, as he was in “Tropic Thunder,” with more structure-minded screenwriters, so that the idiosyncratic detours have a smooth highway to return to when needed.

Showing at: Brunswick 10, Cinemagic Grand (South Portland), Cinemagic Saco, Cinemagic Westbrook, Falmouth 10, Nickelodeon (Portland)