Christmas, 1669. In the grip of the coldest winter for years, the River Thames is frozen from bank to bank and London celebrates with a traditional frost fair held on its broad back. Revellers come from far and wide to enjoy the spectacle: an ox is roasted, booths set up and entertainers employed to amuse the crowds enjoying the holiday atmosphere. Among the throng is ambitious young architect Christopher Redmayne, escorting the daughter of one of his clients with whom he hopes to further a romantic attachment. By chance they meet Christopher's good friend, Constable Jonathan Bale. When a child slips on thin ice the pair make a chilling discovery of a frozen naked corpse embedded in the ice.