In Calcutta, India, Scotland Yard Inspector Lewis Stone Marney teams up with local Commissioner Matthew Boulton Grimshaw to unpack the murder of Leonard Lee, as they approach the bungalow where Lee was stabbed in the back. Grimshaw bluntly sums up the victim as “rotten.”
Inside the supposedly locked house, Henry Daniell as John Wales stands as Lee’s best friend, proposing a séance not to summon spirits but to apply psychological pressure on the suspects. The plan is accepted, and Grimshaw telephones the Governor, Sir Roscoe Crosby Holmes Herbert, to arrange the night’s proceedings.
At the Governor’s residence, the gathering is a mix of kin and colleagues: the Governor himself, Holmes Herbert as Sir Roscoe Crosby, his wife Lady Crosby Janet Beecher; their daughter Helen Trent Elissa Landi and her husband Major Lionel Trent Ralph Forbes; their son Dick Crosby Thomas Beck and his mother’s secretary Nell O'Neill Madge Evans, who is secretly engaged to Dick; Dr. Mason Charles Trowbridge; Mary Eastwood Heather Thatcher; Professor Feringeea Lal Chand Mehra; Mr. Stanby Robert Coote and his emotionally unstable sister Miss Stanby Elsa Buchanan; and Wales himself, as well as the medium, Mme. Rosalie La Grange May Whitty.
The medium, a grandmotherly woman with a working-class inflection, begins with a few tricks—tables that float and disembodied knocks—before promising there will be no trickery tonight. While the women step aside to witness a body search, she has a cryptic exchange with Nell. The group seals doors and windows, forming a circle with thirteen people. Following Mary Eastwood’s lead, La Grange is tied to her chair, and guests’ handkerchiefs are linked to emphasize the circle. When the lights go out, the room is dim, and La Grange’s voice is heard alongside Wales’s. Wales repeatedly asks, “Do you know who killed you?” and then falls silent. When the lights snap on again, the circle reveals Wales dead in his chair, struck by a fatal knife wound to the heart.
Inspector Marney locks down the scene, insisting that if Wales spoke truth, the killer must be among those in the room, and so too must the person who killed Wales. Dr. Mason’s caution about the possibility of hidden truths is tempered by the discovery of Nell’s handkerchief on the floor, which Marney shows to him before returning it to her. The murder weapon remains missing. The investigation unravels a tangle of motives and red herrings. Nell reveals that La Grange is her mother, a fact the medium surprisingly accepts, noting only that it changes nothing for the Crosby family’s feelings toward Nell. Nell’s visit to Leonard Lee the night of the murder is brought up, with Helen denying any involvement and Marney eyeing Nell for possible arrest.
La Grange begs for a moment of privacy in the dark, asking the heavens for a true message. The inspector interrupts, but the afternoon’s message turns out to be literal: a knife embedded in the ceiling becomes the focal point of the mystery.
The absence of fingerprints on the knife leads Marney to a new line of reasoning. Nell’s blood-stained handkerchief plus the way hands were moved within the circle reveal how the killer may have acted. The medium’s family ties come into play as Marney tests loyalties and alibis, and the room’s dynamics begin to betray hidden alignments. Dick’s lampblack-stained hand—the result of a deliberate attempt to create a misleading trace—points toward a coordinated effort involving Mason, who notably keeps his hands clean in the outward sense. Yet the crucial clue shows that the perpetrator reached across Dr. Mason, leaving a telltale trace that Marney recognizes as revealing the true culprit.
In a final, quiet accusation, the evidence aligns: Mason kept himself apart from the circle and manipulated the others’ movements to mask his own role in Wales’s death. The circle’s physiological tells—the way hands moved, who touched whom, and who reached across whom—uncovers the truth that Wales never suspected this killer for a moment. Ironically, the killer’s careful placement and the room’s order expose the murderer as someone who believed they could control every variable, only to be undone by a single, incriminating detail. The case closes with Marney laying out the deduction: the killer was Dr. Mason, whose clean hands belied a calculated obsession with concealing his crime.
