Look, up on the screen–it’s a bird, it’s a plane, it’s a lame superhero comedy. Well, actually that’s not entirely fair. Despite the fact that one of the main characters in it is a Superwoman type called G-Girl, “My Super Ex-Girlfriend” is really no more than a formulaic scorned-woman-strikes-back romantic comedy in which the vengeance-seeking femme just happens to possess powers and abilities far beyond those of mortal women. All the super-hero twist does is to allow her wrath to take more outrageous forms than usual.

But otherwise the script pretty much follows the numbers. Matt Saunders (Luke Wilson) is a likable schlub who works in an architectural firm. He’s got a pushy boss (Wanda Sykes) with sexual harassment concerns on the brain and the obligatory wacko best friend, Vaughn (Rainn Wilson, Dwight from “The Office”) who’s constantly giving him bad advice on women. And he has a bright, pretty desk-mate, Hannah (Anna Faris) with whom he’s smitten, but who has a studly boyfriend (Mark Consuelos). At Vaughn’s urging Matt approaches a young woman on the subway one day; she happens to be Jenny Johnson (Uma Thurman), the alter-ego of the fabulous G-Girl, and though she’s initially standoffish, when Matt tries clumsily to retrieve her purse from a snatcher she decides to give him a chance. The two go out and though she’s strangely demanding and has a habit of disappearing with some frequency, the lovemaking proves so powerful that he’s willing to set his misgivings aside and then become a couple. Soon, though, he finds it impossible to put up with her clinging, jealous behavior, and anyway Hannah has broken up with her guy and is now available. So he tries to break up with Jenny, leading to G-Girl’s revenge. Things get so bad that Matt links up with her arch-enemy, Professor Bedlam (Eddie Izzard), a guy whom she jilted back in high school, to take away her powers. Of course, nothing goes as planned, and there’s a big, extended finale which sorts the couples out just as they should be. But it’s made abundantly clear that this is as much fantasy as comedy or romance, not only because super-powers are displayed all over the place, but because one of the climactic elements is a cat fight over…Luke Wilson?

This all sounds pretty bad, but it isn’t quite as awful as it could be. Wilson may not be the most charismatic leading man on the planet, but he has a low-key, rumpled charm, and though Rainn Wilson’s playing a cliche, his droll delivery gives his lines more punch than they’d otherwise deliver. For most of the way Faris is a plus, too, sprightly without being irritating about it. And Izzard keeps Bedlam nicely toned down, although the comic business involving his inept henchmen (Stelio Savante and Mike Iorio) gets old pretty fast. The cheesy visual effects fit the bill, too, and if the constant strain of sniggering sexual humor gets more than a little tiresome, that’s to be expected in any comedy Hollywood produces nowadays.

But even through the first two-thirds of “Ex-Girlfriend” there are problems. Ivan Reitman’s direction is pretty limp, offering little point or edge to the proceedings. (His son displayed a much more developed comic touch in “Thank You for Smoking.”) And though Thurman has Jenny’s abrasively defensive pose down pat, she fails to instill even the slightest trace of likableness into the character. The script never bothers to suggest why she turned out the way she did, anyway, which leaves the actress with little to do but pout and bristle in a vacuum. In one scene, when Jenny’s reluctant to leave a meal to save New York from a runaway rocket, her air of mono-maniacal exasperation pays real dividends, but elsewhere the one-note nature of the performance just makes the woman seem mean and petty, without the saving touch of niceness that would make you forgive her bad temper. The same applies to her acts of revenge after Matt dumps her. One of them, involving a “Jaws”-like cameo, has a gleefully twisted tone, but otherwise they’re actually quite dull.

Still, it isn’t until the big finale that brings all the characters together in chaotically farcical action mode that the movie really collapses. Reitman and first-time screenwriter Don Payne (whose previous work has been limited to “The Simpsons”) obviously felt that the movie needed a really splashy conclusion, and so they go into full “Ghostbusters” mode. But a bruising fight between two super-powered people is actually unpleasant, not funny, and the final wrapup lacks the breezy sense of contentment it needs. The animated bits that accompany the final credits lack pizzazz, too.

“My Super Ex-Girlfriend” isn’t terrible; for the first two-thirds or so it has some shambling charm, especially in the scenes involving the two Wilsons; and though one might have hoped for more genuinely acerbic wit from a script by a “Simpsons” writer, there are some good lines. Things begin to deteriorate when the woman-scorned plot kicks in, though, and Thurman just isn’t an inventive enough actress to generate any sympathy for what’s really an unlikable character. And the last thirty minutes turn loud and curiously unpleasant. One can imagine the movie having worked better either as a really nasty, edgy satire or in sweeter mode, as sort of a super-heroine variant of “Splash.” But simply grafting the special-effects scenario onto a standard-issue spoiled relationship plot makes for no more than a tolerable but mediocre big-screen sitcom that crash-lands without ever really having taken off. Or to put it in a way the material demands: if it were a comic, it would be cancelled after three issues.