The story involves a fake psychic, Blanche Tyler (Barbara Harris), and her con artist taxi driver boyfriend, George Lumley (Bruce Dern), who attempt to locate the nephew of a wealthy and guilt-ridden old woman, Julia Rainbird (Cathleen Nesbitt). Julia, one of Blanche's clients, was responsible for her now-deceased sister giving up a boy for adoption years earlier and now wants to make him her heir. She will pay Blanche $10,000 if he is found.
However, the nephew and prospective heir is now a successful jeweler in San Francisco known as Arthur Adamson (Williams Devane), who has a secret and lurid past, having apparently murdered his adoptive parents and faked his own death. With his girlfriend Fran (Karen Black), he has successfully kidnapped an assortment of millionaires and dignitaries, returning them when the ransom, a valuable gemstone, has been delivered, which they hide in their chandelier.
Eventually the paths of these two couples cross when Adamson learns that Blanche and George are pursuing him, and he suspects the worst, putting their lives in danger. When George begins to research the history of the missing nephew, he learns about the original name of Adamson which was Edward Shoebridge and he follows the leads to a gas station owner named Moloney (Ed Lauter) whom helped Adamson with faking his death and covered it up. Adamson then sends Moloney to kill both Blanche and George by pretending that he knows information about the missing nephew and agrees to meet at a remote restaurant in the mountains. Moloney attempts to sabatage the brakes on George and Blanche's car while they wait at the restaurant for Moloney to show up. When he doesn't, they leave and find themselves trapped in their car as it skids town the sloping mountain road. But they manage to survive. Failing this, Moloney attemps to run them down on the road, only to have his own car go over a cliff and crash... killing him.
George learns from Moloney's wife about his part in the Edward Shoobridge murder/coverup and learn the alias of Arthur Adamson. While George has to take a shift at driving his taxi, Blanche decides to look for Adamson on her own. Finally tracking down Adamson to his jewlery store which he owns, she goes over to his house where he and Fran are in the process of delivering back another kidnapp victim, a wealthy Catholic bishop named Wood, when they are about to pick up their latest ransom.
Adamson and Fran trap Blanche in the basement garage of their house when she comes to tell them the good news about the Rainbird inheritance. Seeing that Adamson is a criminal responsible for the kidnappings in the area, Blanche attempts to flee, but they capture, drug her, and lock her in their secret room in the basement where they keep their captives.
A little later, George, learning about Blanche's discovery, heads over to the Adamson's house and after finding nobody home, he notices Blanche's car outside. George sneaks into the house through a basement window. When Adamson and Fran return after collecting another diamond for this latest Bishop Wood ransom, George hides and follows them down into the basement's secret room where he overhears them ploting to kill Blanche. George and Blanche manage to turn the tables on the other couple and suceed in locking them in their own secret room.
Afterwards, Blanche is elated when she thinks about how much the reward will be for catching these thieves, until George points out how much more it will be if they could also locate the stolen diamonds. Suddenly, Blanche seems to go into a trance and makes her way to the crystal chandelier on which the diamonds are hanging. George congratulates Blanche for really being a psychic. As George picks up a phone to call the police, Blanche smiles and winks at the camera.
