When a space mission involving American and British astronauts encounters an alien craft, the humanoids within are brought aboard the shuttle. Back on Earth, one of the extraterrestrials, who appears to be a gorgeous woman (Mathilda May), proceeds to suck the life force out of various Londoners, turning the town into a city of roaming half-dead people. When Tom Carlsen (Steve Railsback), a surviving astronaut, realizes what is happening, he sets out to stop the ruthless alien presence.