Storm Harvest (Doctor Who Series)

Years after the extinction of its indigenous races, the sea-world of Coralee is populated by human and dolphin colonists. They are ignorant of what caused the original population to die out, but tales of vicious monsters proliferate.

An underwater massacre makes the Doctor and Ace wonder if one aspect of the legends, the acquatic Krill, might be real after all -- and if so, if they are as deadly as the stories say.