An armada of extraterrestrial spaceships hovers over the major cities of Earth. The humanoid aliens called the 'Visitors' claim they came in peace to seek human help, but soon strange things begin to occur. Cameraman Michael Donovan secretly goes on board one of their ships to investigate.

A resistance movement is formed in Los Angeles under the medical scientist Julie Parrish. Visitor dissidents opposing their leader's plans join their cause to fight the alien oppression by leader John and his right hand Diana.