Roger, Diana, Snubby and Barney just love solving mysteries - with Loony the dog and Miranda the monkey to help!
Miss Pepper hopes that taking the children to stay at the village inn in Rubadub will give them all a quiet holiday - for a change. But all sorts of strange things are going on at the inn, and there are mysterious explosions at the secret submarine base nearby. Who is behind the happenings at the inn? The police are on the trail, but the children are determined to find the answer first!