A zombie movie with humorous overtones is hardly a new idea, but this British slacker takeoff on George Romero’s “Dawn of the Dead” (at least insofar as the title is concerned–in many respects it’s more a spoof of the recent “28 Days Later”) adds an unusual element: it wants to be a romantic comedy, too. The bizarre idea’s actually not a bad one, but unfortunately the execution is too inconsistent to make the most of it. “Shaun of the Dead” is easygoing, lackadaisical and rather flat; it needs the fizz and panache of one of Peter Jackson’s or Sam Raimi’s comic horrorshows, but comes across as kind of bland instead. Watching it isn’t a terrible chore, but it’s not the gut-busting mixture of hysteria and bloodletting the premise promises, either.

The titular “hero” is a none-too-swift clerk in a London electronics shop, played by British television personality Simon Pegg. Shaun lives in a house with Pete (Peter Serafinowicz), an up-tight yuppie, and Ed (Nick Frost), a gross and shiftless layabout who’s been his best buddy for years. Somehow the dog-faced Shaun has acquired a perky, long-suffering girlfriend named Liz (Kate Ashfield), who–along with Di (Lucy Davis) and David (Dylan Moran), the judgmental couple she shares an apartment with–urges her guy to spend some time away from Ed and the local pub in which they waste most of their evening hours. Before the story can get very far along the conventional “triangle” path, however, a zombie plague of unspecified cause descends upon England, and after finally comprehending what’s happening (they’re a fairly oblivious pair), Shaun and Ed venture out to rescue Liz (and Di and David along with her), as well as Shaun’s mum (Penelope Wilton) and stepfather (Bill Nighy) by leading them through the dangerous streets to the supposed safety of the pub. There a final showdown occurs.

Amusing moments pop up periodically in “Shaun of the Dead.” The first encounter that Shaun and Ed have with the undead in their backyard is raucously chaotic, and Shaun’s befuddled entrance into Liz’s apartment to “rescue” her (actually removing her from a place that appears much safer than where they’re going) has a farcical kick. Nighy brings some subtle twists to the stepfather figure with whom Shaun has issues even before the bloke turns into a zombie (what might have been a stock villainous turn morphs briefly into something deeper than that), and there’s a fine scene–probably the sharpest in the picture–when Shaun’s little band bumps into another, composed of people who are like oddball reverse-mirror images of themselves. The latter sequence is an especially good example of the characteristically chatty, low-key humor that the British use so very effectively, but most of the other like moments here don’t register nearly as well. The fact is that although the picture is only a bit over an hour and a half long, it grows rather repetitious over that span, and one is likely to find the voluble Frost, in particular, tiring as the story runs on.. (Of course I could be wrong: viewers certainly embraced the similarly oversized John Belushi of “Animal House.”) “Shaun” doesn’t stint on the second half of the equation, either–the guts and gore stuff–but that’s all dished out in such an over-the-top fashion that while you might be somewhat disgusted, you’re hardly going to be frightened.

In the final analysis “Shaun of the Dead” is rather like a sketch on a TV variety show–probably the one called “Spaced” that the filmmakers did in Britain before moving on to this–extended (or more properly overextend) to feature length. The cast is game and there are passing moments of inspired lunacy, but as a whole the movie never builds up a full head of comic steam. It may want to be, as the British might say, a bloody riot, but in fact it seems to suffer from a case of comedic anemia.