The central puzzle in 'A Case of Identity' revolves around a young woman, Mary Sutherland, who hires Holmes after her fiancé, Hosmer Angel, vanishes without a trace on the very morning of their wedding. The situation seems bizarrely straightforward yet deeply unsettling. Miss Sutherland explains that Hosmer was a quiet, nervous man who always wore tinted glasses, spoke in a whisper, and insisted on communicating only through typed letters, even when they were in the same room. He disappeared from a cab outside the church, leaving no physical evidence behind. From the outset, Holmes perceives the obvious inconsistencies in the tale, focusing less on a conventional disappearance and more on the manufactured nature of Hosmer Angel himself.
Holmes quickly deduces that the mystery isn't a crime of violence but one of cruel deception. His investigation centers on the motives of Miss Sutherland's stepfather, Mr. Windibank. Holmes identifies that Hosmer Angel's peculiar habits—the typed letters, the whispered voice, the glasses—were all a deliberate disguise to prevent Mary from recognizing him. The shocking, yet somewhat morally grey, resolution is that Hosmer Angel was Mr. Windibank in disguise. The stepfather concocted the entire charade to prevent his stepdaughter from marrying and leaving home, thereby ensuring he could continue to control the comfortable income she received from an inheritance. The 'mystery' is ultimately a heartless financial scam rather than a murder or theft, showcasing Holmes's ability to see through emotional manipulation to a cold, logical truth.
The story's intrigue lies in this personal betrayal and the stepfather's cynical exploitation of familial trust. Holmes solves the case with relative ease, but the satisfaction comes from exposing the mundane yet profound cruelty behind the theatrical disappearance. He famously chooses not to bring the stepfather to legal justice, arguing the law cannot touch him and that the exposure itself is punishment enough, leaving Miss Sutherland with the painful knowledge. It's a case where the solution feels simultaneously clever and deeply sad, a reminder that the most damaging mysteries can occur within a family's own drawing room.