Colonel Barclay and his wife have a serious disagreement after she accidentally meets a man from her past while in the company of her friend, Miss Morrison. Upon returning home, Mrs. Barclay, named Nancy, has a loud argument with her husband, the Colonel, behind the locked door of the morning room on which the servants gladly eavesdrop. A cry is heard from the Colonel's lips, a loud crash occurs and Mrs. Barclay screams.
Mrs. Barclay is under arrest for suspicion of murder; Holmes uses the fact to press Miss Morrison into divulging what she knows of the quarrel, even though it means breaking her promise to Mrs. Barclay. The man, the crooked man, Mrs. Barclay encountered, with whom she spoke privately, was a man she and her husband had known before their marriage while living in India during the Indian Mutiny. Barclay had been the crooked man's Sergeant and had asked for a volunteer to summon reinforcements; the volunteer had been the crooked man, one Henry Wood. Wood and Barclay had both been in love with Nancy.
Henry was captured in an ambush and had reason to believe that Barclay had betrayed him to leave the field clear for winning Nancy's agreement to marriage. Nancy learned this at the unexpected encounter with Henry, deformed and made crooked by torture after his capture, and is incensed, accusing the Colonel of the sin of David in regard to Bathsheba (David sent Bathsheba's husband to a surely fatal battle so he might take her as his own wife).
During the argument, Henry, his mongoose with him, comes into the morning room from the garden unbeknownst to the servants. At the sight of Henry, Colonel Barclay suffers apoplexy (a cerebral hemorrhage), cries out and crashes to the floor, dead before his head hit the tile. An inquest confirms the cause of death as natural causes due to apoplexy, and Nancy is exonerated and freed with no shadow of guilt hanging overhead.