Casper is a kind young ghost who peacefully haunts a mansion in Maine. When specialist James Harvey arrives to communicate with Casper and his fellow spirits, he brings along his teenage daughter, Kat. Casper quickly falls in love with Kat, but their budding relationship is complicated not only by his transparent state, but also by his troublemaking apparition uncles and their mischievous antics.

Ambitious television reporter Tim O'Hara stumbles upon a martian 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.