On the last day of the first manned mission to Mars, a crew member of Tantalus Base believes he's made an historic discovery; fossilised evidence of bacterial life. Unwilling to let the relief crew claim the glory, he disobeys orders to pack up, and goes out on an unauthorised expedition to collect further samples. But a routine excavation turns to disaster, when the porous ground collapses, and he falls into a deep crevice and near certain death. His devastated colleagues attempt to recover his body. However, when another vanishes, they begin to realise that the life-form they have discovered is highly dangerous to all human life.