In her work clothes, Jenny Johnson ... it's alliterative, like Clark Kent ... is a slightly nerdy Manhattan gallery employee with smart-girl glasses, mousy-brown hair and a personality to go with it. As G-Girl, a crime fighter who can fly, lift heavy objects, take bullets in the chest and make love like ... sorry, I really can't think of a good simile here ... she's a willowy blonde, although you can tell from her lousy dye job that the drapes don't match the carpet.
One day on the subway, a mugger snatches Jenny's purse just as she's rejecting an offer to have coffee with Matt Saunders (Wilson), whose personality is as generic as his name. Matt runs after the thief, and his bravery impresses Jenny. Soon, they're pounding the walls in bed (Jenny is, to say the least, an energetic lover), and she falls so deeply in love that she reveals her secret identity. Then, Matt dumps her for a sweet co-worker (Anna Faris), and Jenny/G-Girl becomes a super-stalker who takes super-revenge.
The lovesick Lex Luthor of the piece goes by the sobriquet Professor Bedlam (Eddie Izzard). He and Jenny were high school friends and fellow nerds, until a meteor gave her superpowers, and she dumped him. Now he's discovered a way to drain her of those powers, and he finds just the right frightened ex-boyfriend to help him execute his plan.
I suppose there's a movie in here somewhere, but director Ivan Reitman (Ghostbusters) and screenwriter Don Payne (an Emmy winner for The Simpsons) don't come close to finding it. Rather than making a tart satire, filled with trenchant one-liners, they offer only middling gags and an easy-going faux-sophistication. A simpleton like Matt would be lunch in a real New York, and it doesn't help to have him played by an actor with no charisma or range.
Quirky high-concept is fine, but Wilson's frequent collaborator, Wes Anderson (Rushmore and so forth), knows that you also need character, dialogue and intellect to accentuate your quirks. Super Ex has neither, save for a quip here and there. When G-Girl kicks into full jealousy mode, Thurman works awfully hard to take it over the top. But her persona is weirdo, not wacko, and her defining role remains the one she played in Pulp Fiction, where she took a needle in the heart and sprung back to what little life she has on screen.