After a computer selects a group of individuals to survive an impending nuclear strike, they are sealed inside a secure underground shelter. Tensions rise as personalities clash under pressure, but a far deadlier problem soon emerges. The survivors discover they are not alone in the bunker, forcing them to confront a growing and unexpected threat while remaining trapped below ground. The 1974 film blends science fiction with suspense as the group struggles to endure their confinement.