Ambitious television reporter Tim O'Hara (Jeff Daniels) stumbles upon a martian (Christopher Lloyd) whose spaceship has accidentally crashed on Earth. Thinking this is his ticket to a Pulitzer Prize, Tim makes plans to expose the martian, who, posing as Tim's Uncle Martin, takes human form and has plans of his own. In fact, all of Tim's efforts to divulge the truth are systematically thwarted by the resourceful alien, who ultimately recruits Tim to help him repair his ship for a return to Mars.