The changeling: Kirk finds a way to stop a killer computer from its next target --Earth. The apple: The god-like ruler of Gamma Trianguli VI views the Earth as a threat that must be destroyed.

Escaped killers led flee a Nevada prison to the ghost town of Yucca Flats, even though a nuclear blast is set to detonate there at 6 a.m. the following morning. Drawing a hapless group of passersby into their circle of desperation, the killers hold court over a night of suspense and terror.