As in the three previous books of the series, Robert and Diana as well as their cousin Snubby leave for vacation, accompanied by Miss Pepper. Their destination is the seaside village of Rubadub, close to a secret submarine harbour. They lodge in the 'Three Men in a Tub'-Inn, which has a set of interesting guests from the nearby variety show. Things get moving when their friend Barney from the previous three adventures, still in search for his father, arrives. With sabotage at the secret harbour and police coming to the inn, the four are again deep in a mystery. But who is friend and who is foe, and will Barney finally find his father? This is the fourth book in the 'Barney Mystery' series. It was originally intended to be the final one.