The grim dilemma facing two astronauts on the top secret Apollo 18 mission to the moon would stump even the hardiest of space travellers.
Do they (A), stay inside the the safe confines of the lunar landing module and listen to tapes of 70s prog-rockers Yes...or (B) step outside and take their chances with crab-like aliens keen on burrowing into their flesh?
Obviously, the answer is B. I mean, who in their right mind would want to lend an ear to Yes?
Director Gonzalo López-Gallego's atmospheric chiller posits that "found footage" reveals how the mission had a far more disturbing aim than merely taking one huge step for mankind.
Viewing the grainy, camcorder shot film almost fifty years after the event, we watch the two-man crew as they position defence department surveillance equipment on the lunar wastes.
However, all is not well. Communication with their mother ship and Houston Control is regularly mashed by strange interference and there's the sound of something rattling across the ship's hull.
Final confirmation that there's a bad moon rising comes when they find the desiccated body of a Russian cosmonaut and his abandoned, blood-spattered Soviet Lander.
López-Gallego deftly meshes the claustrophobia of being trapped with limited air in either the cramped lunar module or cumbersome space suits with the dawning horror that something else is out there. Or maybe in there with them.
He never overplays his hand: the nasty critters are only half glimpsed on grainy video and the sweating fear of their victims is almost palpable.
It's enough to keep you firmly earthbound.