In December 1377 a great frost has the city in its icy grip; even the Thames is frozen from bank to bank. Murder, revenge and treachery also make their presence felt. The Constable of the Tower of London, Sir Ralph Whitton, is found murdered in a cold bleak chamber in the North Bastion. The door is still locked from the inside and guarded by trusted retainers - so how did the assassins slip across a frozen moat and climb the sheer wall to commit such a dreadful crime? Athelstan and Sir John Cranston, the wine-loving coroner of the city of London, are appointed to investigate these mysteries. They soon discover Sir Ralph's murder is only the first in a series of macabre killings which have their roots in a terrible act of betrayal committed many years previously.