Detective John Hooper, William Monk’s right-hand man at the Thames River Police, is blissfully happy in his new marriage to Celia, the cousin of a victim in one of the river police’s recent murder cases. Celia wants the same happiness for her good friend Clementine, who’s just announced her engagement to Seth Marlowe, a member of her church. Christmas is nearing, and this should be extra cause for celebration, but when Marlowe begins receiving threatening letters about his first wife’s death, it becomes clear that he is far from the devout man Clementine thought he was. In his rage, Marlowe accuses Celia of sending the letters, claiming she wants to ruin his engagement to Clementine. At a loss as to how to defend herself, Celia enlists Hooper to investigate the letters’ claims, and what he finds makes her desperate to show Clementine the truth about her soon-to-be husband.