Underneath the Thames, chained to the banks of the river, is an ancient creature of unknown origin. Alien or not, this giant sea snake has been responsible for the freezing of the Thames at regular intervals – none less so than during the last great Frost Fair of 1814!Imprisoned by the ancestors of Lord Sutcliffe (since their discovery that the animal’s dung could be used as a superheating biofuel) the creature has a symbiotic relationship with a number of glow-fish in the river, which attract food and melt the ice to bring unsuspecting humans down into the water to allow the snake to feed. When the Doctor and Bill discover the creature, they become anxious to alleviate its suffering and find a way to free it… even if it means making a powerful enemy in the guise of Lord Sutcliffe.
The Doctor tricks the iniquitous industrialist by using his explosives to break the ‘Ice Snake’s’ chains, in the process preventing the death of the Frost Fair attendees (evacuated by Bill and the urchins), but as the ice breaks up, Sutcliffe is killed. With the creature freed, 1814 is the final Frost Fair on the Thames.