Nancy, George, and Bess have been invited to an April Fool's Day party at their new schoolmate's house, with each guest bringing a gag to the party and the best prank winning a special prize. When two of the guests' fancy new electronics go missing, Nancy knows it's no joke, and the Clue Crew are on the case to find the missing gadgets.

Famous astronomer Dr. Arnot gives Nancy, Bess, and George a star-studded planetarium show and promises to let them gaze at the night sky with his famous telescope.

A client of Nancy's father is accused of stealing the magnificent spider sapphire and exhibiting it as his own creation. Nancy uncovers a nefarious scheme while working to solve the mystery.