Mike, Peggy, Nora, Jack, and Prince Paul are invited to spend their holidays at the incredible Moon Castle, a majestic fortress set high in the mountains of Baronia. At first, it seems like the perfect royal getaway, but the children quickly realize that the ancient castle is filled with strange and terrifying mysteries. Unexplained glowing lights hover in the dark towers, heavy objects float through the air as if by magic, and strange, rumbling noises echo from deep beneath the stone floors. The local villagers believe the castle is haunted, but the brave young adventurers are determined to find a logical explanation.

Refusing to be frightened away by the "ghosts," the five friends begin a secret investigation that leads them deep into the forgotten, crumbling mines beneath Moon Castle. There, they discover that the strange phenomena are not supernatural at all, but the work of a ruthless gang of criminals exploiting a rare, glowing, gravity-defying mineral hidden in the rock! Trapped in the dark, twisting underground tunnels with the dangerous thieves closing in, the children must use all their courage and cunning to outwit the villains and expose the incredible secret of Moon Castle before it is too late.