Eleven-year-old twins Jason and Julia have just moved from London to an old mansion on the English coast. Their new home is filled with twisting tunnels and strange artifacts from around the world, and the twins can't wait to discover all its secrets. Before long, Jason, Julia, and their friend Rick stumble upon a mysterious-looking door hidden behind an old wardrobe. But none of the keys in the house will open it. What lies behind the door? And why has someone tried to conceal it? Jason, Julia, and Rick are determined to find out, no matter what it takes. . . .