The night after visiting a museum exhibit of "Holy Grails Throughout History," young Rogers Hardy sees a strange light in his backyard. The next morning he discovers the unconscious body of a young girl there--a girl identical to his healthy sister Ginger. Brother and sister nurse the stranger back to health, discovering to their amazement that her outward appearance can change on a whim, and her vocal ability is limited to speaking Latin. Soon they realize she is a young angel who lost the real Grail to the museum collection and is now charged with getting it back. The trio develop a crazy plan to steal it back from the museum during a total eclipse of the sun, all the while hiding their every action from their cancer-stricken mother. Her illness, in fact, is their primary motivation, for it they can get her to drink from the Grail before their new friend leaves, she may regain her good health.