The past few years have introduced a handful of promising young action directors capable of making taut, economical genre films that capture the best aspects of superior B-movies. Like similarly inclined director Jonathan Mostow (Breakdown, U-571), David Twohy has followed his breakthrough film (Pitch Black) with a submarine movie set during WWII. In a rare lead performance, veteran character actor Bruce Greenwood stars as the stoic leader of a damaged submarine rescuing survivors of a sunken hospital ship, including Olivia Williams, a strong-willed nurse whose presence brings the sub’s already-simmering tension to a boiling point. Greenwood’s ship is seriously damaged by enemy attacks, but the crew’s dark secret poses as much of a threat as the German U-boats. Twohy and company keep dropping hints of the macabre into what’s otherwise a tense and atmospheric thriller, and for much of its duration, Below deftly toys with audience expectations. Is this a submarine film, a ghost story, or some genre-blurring combination of the two? Twohy and co-screenwriters Darren Aronofsky and Lucas Sussman don’t show their hand until late in the film, but by that time, Below has grown slack and silly. For an hour or so, it suggests Twohy’s Pitch Black gone underwater, as a motley group of tough men and women confront a mysterious common enemy. But just when the staccato jolts of horror should be building to a terrifying climax, the film deflates, and its atmospheric opening gives way to a flabby final act characterized by wooden dialogue and a thuddingly anticlimactic final twist. Below begins as a superior B-movie that makes memorable use of silence, the ship’s claustrophobic confines, and a Benny Goodman song whose shrill peppiness takes on a mocking, sinister quality when contrasted with the crew’s mounting paranoia and fear. In the end, Twohy’s intriguing but disappointing film can’t commit to being either a horror movie or a submarine thriller. As a result, it works as neither.