Having spent some time by the river watching the boats go by, Mr Benn sets off for the costume shop and tries on a frogman’s suit. On leaving the changing room he arrives on a rocky shoreline, but when he dives in and begins to explore the water, he is soon interrupted by the crew of two submarines, who are each searching for a sea monster. Mr Benn then encounters a beautiful mermaid, and helps her to find a seashell for her undersea kingdom’s ruler. Paying a visit to King Neptune, Mr Benn learns that the monarch and his pet sea monster are no longer able to go out because of the nuisance the two submarines are causing in their attempts to photograph the creature. Mr Benn comes up with a cunning plan to trick the sailors into believing they have seen the monster, so that they will then go away…
