A stone-age community live right next to a dusty and dangerous road, frequented by dinosaurs and other prehistoric animals, but want to live in the fresh green countryside. There's just one problem: there are no caves. Mr Benn has the answer; they can build stone huts.

Mr Benn helps King Neptune and his mermaid friend outwit the crews of two submarines, who are out to find and photograph the King's pet sea monster.