A movie about two alienated high-school kids who find what appears to be the corpse of a naked woman in an abandoned building and, finding that she’s not dead but “undead,” make her their sex slave, doesn’t frankly sound very promising; you approach it with the phrase “schlock alert!” planted firmly in mind. But “Deadgirl” turns out to be a pleasant—or I should say pleasantly unpleasant—surprise. It’s a picture that doesn’t ignore genre conventions, but uses them to spin a tale more about teen angst than simple horror effects. It’s hardly perfect—some of the acting’s pretty awful, and the last reel goes a bit off the rails. But compared to the drek that Hollywood’s horror machines keep pouring out, this weirdly unsetting, and sometimes touching, piece is a winner.

Directors Marcel Sarmiento and Gadi Harel, working from a screenplay by Trent Haaga, get an unexpectedly textured performance from Siloh Fernandez as Rickie, the more sensitive of the two boys who venture into the basement of a deserted asylum and discover the woman (Jenny Spain) in a plastic tarp, bound and apparently murdered. When she turns out to be animate, though also predatory, Ricky’s companion in outsider-ness, JT (Noah Segan) decides to take advantage of their find to meet his sexual needs, but Ricky demurs. He gets even more antsy when the increasingly out-of-control JT introduces another pal, a doofus named Wheeler (Eric Podnar), to their secret. That mistake leads to the involvement of school bully Johnny (Andrew DiPalma), who’s already brutalized Rickie because he had the temerity to look longingly at the jock’s girlfriend Joann (Candice Accola). His meeting with the dead woman sets off a chain of events that have disastrous results.

If you like, you can take “Deadgirl” as a morbidly black comedy, and there are gruesomely funny moments in it—most notably the scene in which JT and Wheeler, having discovered that the zombie’s bite can turn others, try to abduct a prostitute to start a harem, only to find you shouldn’t mess with hookers. But for the most part the picture’s an acutely painful study of Rickie, a kid without hope but still clinging to a frayed ethical sense. Fernandez plays him with genuine feeling, not only in his interaction with JT, Joanna and Johnny, but in the scenes showing his miserable home life.

That side of the picture is so good, in fact—as is Spain’s intense turn as the feral captive—that it’s a pity much of the rest is hobbled by overwrought acting that sends it toward camp. Segan’s portrayal of JT’s increased derangement wouldn’t have been out of place in “Re-animator,” and DiPalma’s pretty terrible as the jock-bully. But Accola makes a nice heroine, and Podnar’s turn as the dim-bulb but loyal Wheeler may be over-the-top, but it fits.

A good deal of the effect of “Deadgirl” rests on the atmospheric widescreen cinematography by Harris Charalambouse, which belies what must have been an extremely low budget (the long tracking shots in which the camera prowls the underground tunnels beneath the asylum are honestly gripping), and Phillip Blackford’s editing, which isn’t afraid to take things slowly, though in the action moments it’s appropriately swift and abrupt. Effects-wise, the picture is hardly state-of-the-art, and in fact the level of gore is pretty modest compared to the avalanche of blood and innards that fans of torture-porn are accustomed to.

The fact that it’s relatively subdued by modern standards may, in fact, limit the movie’s popularity among the gross-out crowd, at the same time that its storyline turns off more mainstream viewers. That would be too bad, because “Deadgirl” is, despite some weaknesses, a surprisingly effective character study dressed up as a grisly horror movie—and I suspect that it might be beneficial for young torture-porn addicts to have the opportunity to see themselves in Rickie and JT.