Set in Victorian England, private investigative agent William Monk is once again searching for the invisible. He is begged by young Lucius Stourbridge to find his
missing fiancee, Mariam Gardiner, who suddenly and in inexplicably ran from an afternoon croquette party at the Bayswater mansion of her in-laws-to-be. The coachman who drove her that day is found the following day near the path to the cottage of Cleo Anderson. Miriam is arrested for Treadwell's murder. Further investigation reveals that Cleo raised Miriam since the age of twelve. Further, Treadwell has been blackmailing Cleo for several years for taking medicines from the hospital apothecary for her aging homebound patients. Now Cleo is arrested and Miriam is released. Neither Miriam nor Cleo will tell anything, only denying the murdered anyone. Then a third murder occurs, at Bayswater, where Miriam had been released to Stourbridges' care. It is an entwining story and very enlightening of social attitudes and practices of the time. Sir Oliver Rathbone pursues the truth in court with Monk's wife, Hester, a driving force