London, 1672: A vicious killer stalks the court of Charles II, inscribing his victims' bodies with mysterious markings. Are these the random murders of a madman? The deadly consequence of a personal vendetta? Or the grisly result of a hidden conspiracy? Cambridge, 2008: A Trinity College history professor is found dead, the torn page of a seventeenth-century diary in his hand. His death appears to be an accident, but the college's newest Fellow Claire Donovan and historian Andrew Kent suspect otherwise. The professor's last research subject was Hannah Devlin, a physician to the king's mistress and the keeper of a diary that holds the key to a series of unsolved murders in 1670s London. Through the arcane collections of Trinity's Wren Library, the British Library, and the Royal Society, Claire and Andrew follow the clues Hannah left behind, unearthing secrets of the past and present as both stories unfold to their shocking conclusions.