The Three Investigators and the Secret of Terror Castle

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.
The only clue they have is the name Stephen Terrill, a once-famous inventor who lived in what is now called "Terror Castle." The boys soon found out how aptly chosen the name is: doors open by themselves, gates slam shut as if by magic, things move, slide, crash, appear and disappear. Plus there seems to be a ghost! It would all be too much for the hardy trio if it weren't for the ethereally pretty Caroline, a little girl who claims she is a psychic and fells a particularly strong bond towards Bob.
Unfortunately, Caroline is also the daughter of the local sheriff, who shoos them away from Terror Castle. It takes more than a narrow-eyed, ill-tempered sheriff to shake off the three investigators, however, and soon the boys are back in the house, determined to find out what Jupiter's parents were looking for - something that is supposed to be hidden in the house. Fearlessly defying the mysterious machines that seem to make the house itself come alive, they penetrate to the secret heart of Terror Castle and come upon Stephen Terrill's invention: a diamond-making machine! But there's no time to admire the stones, for the ghost of Stephen Terrill appears - and he doesn't like intruders...