December 1669. During one of the coldest winters for years, the Thames freezes over and a frost fair is held on the thick ice. Architect Christopher Redmayne and puritanical constable Jonathan Bale are both visiting the fair when one of Bale's sons gets into trouble on thin ice. The rescue the boy but in the process make a grim discovery - the frozen corpse of a man. The dead man is Jeronimo Maldini, an Italian fencing master who has been missing for some time. Redmayne is inclined to dismiss the case and leave the investigation to Bale; but all that changes when his own brother, Henry Redmayne, is charged with the murder.