The film follows a hypothetical proposition: a group of astronauts are circling the earth in a spacecraft, but they cannot return, as our planet has become uninhabitable. The cause of this remains open; all-out war, outbreak of a new disease beyond control, radiation after the complete disappearance of the ozone layer, or whatever. The crew of the spacecraft has to find a more hospitable place out there in space, and releases a probe from their cargo bay, Galileo. But Galileo -- after sending back very disquieting data -- has to be sent on a suicide mission.