A team of astronauts led by Mike Hamilton (John Richardson) is sent to investigate an unstable planet, which turns out to be inhabited by a race of green people under the control of an evil computer. The astronauts pledge to help the inhabitants of the planet escape their enslavement and succeed in destroying the computer. Unfortunately, the ensuing explosion takes out the planet as well. The spaceship crew and one surviving alien set off for Earth only to face further difficulties when a member of the crew is possessed by the computer and goes on a killing spree.