First, it's the return of fan-favorite comics companion, space bassist Hattie, as the Twelfth Doctor takes her for the best fish and chips in the galaxy, in a sleepy seaside town in the 1970s.

But there's something ancient and evil beneath the waves, something that has mired its twisted tentacles into the local people, something that weaves itself into Hattie's dreams and drags itself up onto land in mounds of shambling seaweed...

Can the Doctor and...