Geronimo, Thea, and their cousin Trap explore an old castle built by Grant Gentlemouse, the founder of New Mouse City, and when they find one hundred keys and only ninety-nine doors inside, they set off in search of the missing room.

Geronimo is invited by his friend Creepella von Cacklefur to her family's spooky castle, and he is surrounded by the gloom of the castle as well as by Creepella's creepy family!