The setting this week is Regency London, where the townspeople are enjoying a ‘frost fair’ on the frozen Thames. But something very sinister is going on beneath the revellers’ feet!
People are disappearing, sucked under the ice by weird glowing fish and being fed to a gigantic serpent lying on the river bed. Luckily, The Doctor and Bill happen to be in town, so they can look into the mystery.
Teaming up with a group of street urchins, the pair eventually cross paths with a nasty aristocrat, Lord Sutcliffe, who gets a well-deserved wallop from the Time Lord.
The enormous serpent, however, is a different beast altogether – and it will take more than a well-aimed punch to deal with it…