The siblings Kate and Ben and their friend Spider are fascinated by an old mansion called 'Castle House' on the far side of the river next to their village. The mansion has been uninhabited for 15 years, but when they explore the premises they notice a mysterious coming and going. When the old owner, the friendly Mrs. Markham suddenly appears - and as suddenly disappears again - things become even more mysterious. It almost seems as if someone is using the old mansion to hide their criminal activities - but who and what kind of activities? The three friends are determined to find out.