A future Earth, ravaged by nuclear war with aliens. The human race has escaped and moved to Titan, one of Jupiter's moons. A few people have stayed behind in the Tetrahedron to help repair crews on the ground guard the water reclamation machines from alien scabs left behind on the planet. Or are they? Hmmmmm! This is a great film, with slowly revealing mysteries leading up to the ultimate twist. It unfortunately drops a few obvious clues too early on, and I figured out what was going on about 30 minutes in. However it still kept me watching and the action/mystery sections are well mixed keeping the film interesting. A great turn from Mr. Cruise.