Barney, Roger Diane and Snubby love solving mysteries, with the help of Loony the spaniel and Miranda the monkey. Miss Pepper hopes that taking the children to stay by the sea 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 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!