The Doctor is washed up - literally - on an alien beach with only intelligent crabs and a madman for company.
How can he possibly rescue Peri who was lost at sea the same time as he and the TARDIS? But Peri has problems of her own. "Rescued" from drowning by an intelligent sponge growth, she has been adopted by the life form as its own personal God.
As the denizens of the beach come under increasingly vicious attack, the Doctor must discover the truth in time to save all their lives.