The Sea Devils compete with each other to win the flippers that belong to Dane's father. Everyone knows how badly Nicky wants them, so they each disparage their own efforts in the competition, but it turns out that Nicky really does win the contest when he saves everyone's life when a whale smashes their boat.
The Sea Devils agree to help Mona Moray locate her missing father. They find that the professor has been taken captive by a race of undersea aliens who are planning a larger invasion of Earth.