When a series of shipwrecks leaves no survivors, The Doctor realises an ancient race of aquatic creatures called the Sea Devils are responsible. After awakening from millions of years in hibernation, The Master plans to help the pre-historic monsters reclaim the Earth, and wipe out humanity. The Doctor, meanwhile, believes humanity and the Sea Devil can live in peace, but not everyone agrees. Cast Jon Pertwee (The Doctor), Katy Manning (Jo Grant), Roger Delgado (The Master),