Aliens who appear human arrive on Earth in a fleet of 50 huge saucer-like motherships, which hover over key major cities. They appear to be friendly, seeking the help of humans to obtain chemicals needed to aid their ailing world. In return, the Visitors promise to share their advanced technology with humanity. The governments of Earth accept, and the Visitors gain considerable influence with authorities.
However, strange things are soon noted. Scientists find themselves facing increasing media hostility, and government restrictions on their activities and movements. Others, particularly those keen on examining the Visitors more closely, begin to disappear or face being discredited. Noted scientists confess to subversive activities; some exhibiting other unusual behavior, such as suddenly demonstrating an opposite hand preference to the one they are known to have.
Journalist and TV cameraman Michael Donovan (Singer) sneaks aboard one of the Visitors' motherships and discovers that beneath their human like facade, the aliens are reptilian in nature, and carnivorous - preferring to eat live food, like rodents and birds.
Donovan gets some of this on tape, but just as this exposé is about to air, the broadcast is interrupted by a Visitor announcement, suddenly making Donovan a fugitive, pursued by both the police and the Visitors.
The scientists are persecuted - both to discredit them (as the part of the human population most likely to discover the Visitors' secrets) and to distract the rest of the population with a scapegoat upon whom they could focus their fears. Key human individuals are subjected to a special mind control process called "conversion", which turns them into the Visitors' pawns, leaving only subtle behavioral clues to this manipulation; others are subjects of biological experiments, including human-alien cross breeding. Some other humans, on the other hand (including Mike Donovan's mother), willingly collaborate with the Visitors, seduced by their power.
A resistance movement is formed, determined to expose and oppose the Visitors as much as possible. The Los Angeles cell leader is Dr. Juliet "Julie" Parrish. Donovan is later assimilated into the group and, sneaking again into a mother ship, he learns from a Visitor named Martin that the story about the needed chemicals is merely a diversion. The true purpose of the Visitor's trip to Earth is to conquer and subdue the planet, steal all of the Earth's water and harvest the human race as food, leaving only a few as slaves and soldiers/cannon fodder for the Visitors' wars with other alien races. Donovan also discovers there are dissidents among the Visitors (later known as the Fifth Column) who oppose their leader's plans. Martin promises to aid the Resistance, and gives Donovan access to an alien ship, which he quickly learns how to pilot.
Soon, the members of the Resistance strike their first blows against the Visitors, while procuring laboratory equipment and weapons to carry on the fight.