Dr. Watson (Nigel Bruce) serves as the resident physician at Musgrave Manor in Northumberland, a stately house that also functions as a hospital for servicemen suffering from shell shock. The estate’s atmosphere is quiet but charged as Sally Musgrave (Hillary Brooke) reveals a soft, personal interest in Captain Pat Vickery, an American fighter pilot recovering there. Her brothers Geoffrey (Frederick Worlock) and Phillip (Gavin Muir) react with immediate dismay, sensing trouble in the family’s arrangements and in the hospital’s delicate balance.
When Dr. Sexton, one of the physicians at the estate, is attacked during a walk, Watson calls in his famous ally, Sherlock Holmes (Basil Rathbone), to bring clarity to the unsettling events. Holmes’s arrival coincides with tragedy: Geoffrey Musgrave is found dead, and Inspector Lestrade (Dennis Hoey) arrives to take charge, quickly arresting Captain Vickery on suspicion of murder. Holmes questions the arrest and begins a careful, methodical inquiry, convinced that the captain may be innocent and that the truth lies deeper within the Musgrave household.
As the investigation unfolds, Phillip is appointed the new head of the estate, aided by his sister, but the turnover is short-lived. Within a single day, Phillip himself is murdered, his body discovered in the trunk of a car. The motive and the method point inward, and Lestrade suspects the recently fired butler, Alfred Brunton (Halliwell Hobbes), though the true danger lies far more insidiously within the family circle. When Lestrade attempts to arrest Brunton, he becomes entangled in the estate’s secret passages, underscoring the labyrinthine nature of the case.
Holmes and Watson turn their attention to the enigmatic Musgrave Ritual, the family’s age-old method for appointing a new head. The clues lead them to Sally’s room, where the ritual’s words appear in a hidden form, revealing that the process is tied to a grand game: a giant chess match laid out across the main hall floor. The living staff and servicemen must act as the game’s pieces, and each move reveals a new layer of meaning about loyalty, lineage, and the family’s fate.
The ritual game directs them to a buried crypt beneath the house. There, Brunton’s body lies with a case containing an old document. Holmes meticulously reads the surroundings and stages a calculated trap for the killer. After others retire for the night, he slips back into the crypt, waiting for the murderer to reappear. Sexton emerges, and Holmes confronts him with the evolving evidence. Yet Sexton overpowers Holmes, seizing his revolver and confessing his role in the killings. The confrontation ends with Sexton narrowly escaping a fatal shot, only to be confronted by Lestrade and Watson who are ready to seize him.
Holmes explains that Sexton had uncovered an old land grant that would have enriched the Musgraves to a fortune of millions. By murdering Geoffrey and Phillip, Sexton aimed to make Sally the heiress and to secure a marriage proposal to her by framing her sweetheart, Vickery. Sally, however, rejects the prospect of profiting from such bloodshed and destroys the document, cutting off the danger at its source and denying Sexton the fortune he sought.
As the case closes, Holmes and Watson drive away, contemplating the moral shift around them. In a reflective moment, Holmes comments on a broader change in society, captured in a memorable line:
> There's a new spirit abroad in the land. The old days of grab and greed are on their way out. We're beginning to think of what we owe the other fellow, not just what we're compelled to give him. The time's coming, Watson, when we shan't be able to fill our bellies in comfort while other folk go hungry, or sleep in warm beds while others shiver in the cold; when we shan't be able to kneel and thank God for blessings before our shining altars while men anywhere are kneeling in either physical or spiritual subjection.... And God willing, we'll live to see that day, Watson.
