Clowns are usually the last people you'd suspect of committing crimes, so Jupiter, Pete and Bob - The Three Investigators - hardly pay attention when a clown sneaks into Jupiter's birthday party and tries to make off with something. Though they don't catch the clown, they retrieve his booty: it's a tape of Jupiter's parents, who died several years earlier. The only thing that's clear in the garbled tape is that Jupiter's parents were also investigators, and on the trail of something big... Jupiter vows to solve the mystery his parents were working on. Theonly clue they have is the name Stephen Terrill, a once-famous inventor who lived in what is now called "Terror Castle."