B

Though it’s being sold as a movie that will offer surprise after surprise, “The Cabin in the Woods” actually reveals its major secret pretty early on, and when the explanation for it comes, it’s one that only a devotee of H.P. Lovecraft could really love. Still, as a take-off on genre convention it’s amusing, even if as a horror movie it proves rather mild, unless you’re revolted by great splatters of blood comically delivered.

The picture begins by juxtaposing two plot threads. One involves a couple of lab technicians, Sitterson (Richard Jenkins) and Hadley (Bradley Whitford), who preside over a mysterious bank of television monitors and control panels in a massive underground installation. The second is a semi-parody of the start of every college-students-meet-backwoods-serial-killers movie ever made. A couple, jock Curt (Chris Hemsworth) and sassy Jules (Anna Hutchison), head off in their RV with three pals for a weekend at a remote cabin. Their companions are equally predictable: Dana (Kristen Connolly), the virginal good girl; Holden (Jesse Williams), the black transfer student befriended by teammate Curt; and Marty (Fran Krantz), the drug-loving wiseacre with a vaguely Screech-like voice. After a typically menacing encounter with a surly hayseed (Brian White) at the inevitable run-down truckstop, they reach their destination, where strange occurrences quickly mount—odd spying devices, an old diary with some portentous passages, and most notably the appearance of a family of zombies that set upon them.

Since the revelation comes so early on, it’s not much of a spoiler to disclose that what’s happening at the lab is closely connected to what’s happening at the cabin. The larger question is why—and scripters Joss Whedon and Drew Goddard dole out the answer in tidbits through to the end. It will come as little surprise to admirers of Whedon’s habit of putting a twist on genre cliche that that’s what he’s up to here, adding a spin to this teens-in-danger chestnut similar to the one that Wes Craven gave to the slasher movie in “Scream.” And in the process he introduces a big end-of-days hullabaloo that also features a special guest-star.

It’s all ghoulish good fun, a mash-up that ties the trashy teens-in-peril archetype to a global conspiracy scenario that allows for lots of CGI effects and bloodletting, all delivered with tongue planted firmly in cheek Watching the five victims get whacked in traditional gory fashion is nicely set beside the goofy business in the surveillance compound, which Jenkins and Whitford play with the practiced assurance of old pros (their conversation with White is especially amusing). The picture also takes jabs at other types of horror flicks, like Japanese ghost stories with wizened specters threatening schoolchildren. And, of course, the heavy dose of hipster irony is a Whedon trademark element.

The question raised by “The Cabin in the Woods,” which is smoothly directed by Goddard and slickly produced (Peter Deming’s crisp widescreen cinematography abets the elaborate effects overseen by Joel Whist, Scott Treliving and Todd Shifflett), is how fans might react to what the movie implicitly says about them. No less than “The Matrix” and other pictures like it, this is a tale based on the premise that we’re being manipulated by powerful forces with secret agendas. In this case, though, it’s precisely the devotees of the sorts of movies this one is sending up that are targeted as the enablers, or better the consumers, of that manipulation. Of course, as “The Matrix” also showed, audiences either don’t notice such unhappy truths about themselves (or don’t mind them). So perhaps it won’t be a problem in this case either.

And Whedon and Goddard’s film certainly manages to put a clever new spin on a moth-eaten horror genre, although one hardly as revolutionary as the pre-release buzz would suggest.