How does a nightmare begin? For architect David Vincent (Roy Thinnes) it begins when, alone at night on a country road, he witnesses the landing of a UFO. He has stumbled upon a terrifying secret. Aliens from a doomed planet are infiltrating Earth! He attempts to warn the sceptical world, but no one believes him since the aliens look just like us. Only a stiff fourth finger, a lack of a heartbeat, and a tendency to glow give them away. An intriguing combination of paranoid science fiction and dramatic thriller, this highly influential series focuses on one man's determined efforts to save the world.