The book opens in a dimly lit restaurant where the famous actress Mary Blake sits with Donald Morris, a man of high social standing who is deeply in love with her. Mary is tense, overwrought, and wrapped in an impenetrable reserve about her past and her sister Anne. She refuses to talk that night, pleading exhaustion, and Donald escorts her to her apartment on Waverly Place, where they share a single, passionate moment before she flees upstairs.
Following two sleepless nights of agonizing indecision, during which Donald's love overcomes all worldly considerations, he receives a strange, unstamped letter from Mary. Alarmed by its contents, he rushes to her apartment, only to find it completely deserted, rifled by an apparent burglar, and bearing an ominous, dark red bloodstain on the floor and on Mary's filmy silk scarf caught in the door.
Unable to persuade the precinct captain to look beyond a simple case of looting, Donald calls in Peter Clancy, a brilliant private investigator and former secret service operative. Clancy immediately recognizes that the burglary is a clumsy camouflage. A meticulous search reveals subtle clues: a small piece of ice in the kitchen sink that fixes the time of departure, a kitchen window broken from the outside with all the glass resting externally, and missing clothes that suggest only the sister, Anne, has left with luggage.
Clancy's investigation leads him from a nearby garage to a taxi driver who transported a veiled woman with a heavy trunk from Waverly Place to the Pennsylvania Station on Sunday afternoon. Piecing together information from Mary's manager, Frederick Jones, and an old bank account, the detective uncovers the sisters' financial arrangements and a joint fund. Meanwhile, Donald is half-mad with worry, desperately imploring Clancy to find Mary at any cost.
Clancy's relentless search takes him to Hobart Falls, New York, the sisters' remote mountain hometown. There he meets an eccentric old photographer named Walter Lord, who unlocks the mystery of their childhood. He learns that the twins are actually Rosamond and Anne Curwood, daughters of a blind actor named Winthrop Curwood. Lord recounts the dark history of their mother's tragic encounter with a vagrant, which left Anne scarred with a terrible birthmark, and Rosamond's subsequent disappearance years later after a reckless summer romance.
Returning to New York, Clancy discovers that Anne's real life has been entirely shaped by her sister's shadow and her own disfigurement. A midnight stakeout at the Waverly Place apartment finally unmasks the janitor, Angelo, who confesses to a botched robbery attempt inspired by a lottery dream, clearing up the mystery of the bloodstains and the ransacked rooms.
The narrative tightens as Clancy uncovers the final pieces of the puzzle concerning the sisters' true identities and the reasons for their elaborate disappearance. While the detective races against time to bring the two worlds together, the ending hints gently at a resolution where the hidden past is finally brought to light, allowing Donald Morris to face the truth and offer a measure of peace to the woman he loves.
