Ostensibly safely imprisoned on a remote island, the "Master" (Roger Delgado) proves that he can still get up to mischief as the visiting "Doctor" (Jon Pertwee) and his pal "Jo" (Katy Manning) soon find out. It's also around this time that the Royal Navy is having problems with sinking ships. They are bound to be connected, and so swiftly our intrepid time travellers have to find out just what their cunning foe is up to - and with whom! It doesn't take us long to discover that the eponymously ancient species, living in hibernation beneath the waves, have re-emerged bent on reclaiming their plan...