I have to admit that when I first saw the trailer for Spanish director Gonzalo López-Gallego’s entry into the found-footage horror genre, I bought the possibility that his picture might have the right stuff. A top-secret moon mission. A terrifying discovery. Grainy footage of the whole thing shot by the astronauts themselves. What’s not to like?

Well, as it turns out, just about everything other than the film’s premise. For one thing, the director and first-time screenwriter Brian Miller take forever to get their story off the ground. The movie clocks in at less than an hour and a half, and the only reason it’s that long is the shameless amount of padding with which its creators front-load it.

In this case, the launch isn’t delayed by bad weather or technical glitches but by a succession of superfluous touches: faux-documentary-style interviews with the clean-cut crew (Ryan Robbins, Lloyd Owen and Warren Christie); footage of the men being briefed on and preparing for the mission; and faded-color home movies of the astronauts and their families enjoying a beery backyard barbecue. It’s a bad sign when a filmmaker isn’t in a hurry to get to his good stuff.

Some time before the closing credits roll, however, we do lift off. The idea is that it’s 1974, and the Apollo program has officially shut down for budgetary reasons. NASA has unfinished business on the lunar surface, though, so the three men tell their wives they’re going on a routine training exercise and take off for the moon instead. The movie never explains how you launch a Saturn V rocket without anybody noticing.

Once Owen and Christie touch down, we learn why the mission has been kept a secret. Well, we learn, at any rate, that the reason has something to do with the Russians. As in much of the movie, what’s actually happening as the guys go about their business is unclear, owing to a combination of confusing dialogue and an overreliance on scratchy, jumpy video designed to re-create the look of the images beamed to TVs during the real Apollo flights.

In the course of setting up Cold War antimissile systems or radar scanners or creemee stands — it’s hard to tell and hardly matters — the astronauts learn they are not alone. The moon rocks they collect start shape-shifting into lethally infectious spidery deals. One of the two is attacked and begins to show signs of madness, while the other is dismayed to learn the government is thinking of leaving him there to prevent contamination back home.

If all this doesn’t sound terribly interesting or scary, that’s because it’s not. The vérité stuff is humdrum, because that’s the nature of work with lots of downtime. Until its close encounter, the crew makes a lot of extremely small talk while cooped up in the capsule.

The horror stuff is also consistently unsatisfying, because we barely glimpse the little creatures with all the shaky-cam nonsense going on — not to mention the story’s other nonsensical elements. The movie never explains, for example, how a gaggle of sand-crab-sized critters manages to overturn a large lunar rover. Or where these beasties were during the first 17 Apollo missions. Or who’s shooting the footage when both astronauts are in the frame. Houston, we have a turkey.

The cast does a craftsmanlike job, but the meager material keeps it from bringing the characters to believable life. The cinematography is effective enough; the “classified” clips certainly look like ’70s NASA video, but Miller’s script has scant good use for them. As for López-Gallego, let’s give him the benefit of the doubt. Apollo 18 is his first English-language film. Maybe his talent was lost in translation.

The bottom line: In space, no one can hear you ask for your money back.