Dr Watson is called upon late at night by a female friend of his wife. Her husband has been absent for several days. Frantic with worry, she seeks Dr. Watson's help in fetching him home from an opium den. Watson does this, but he also finds his friend Sherlock Holmes in the den, disguised as an old man, trying to extract information about a new case from the addicts therein.
The missing man, Mr. Neville St. Clair, is a respectable and punctual man who makes his family's home in the country but regularly visits London on business matters. Making the matter more mysterious is that Mrs. St. Clair is sure that she saw her husband at a second-floor window of the opium den, in Upper Swandam Lane, a rough part of town near the docks. He withdrew into the window immediately, and Mrs. St. Clair is sure that there is something wrong.
She tries to enter the building, but her way is blocked by the opium den's owner, a lascar. She fetches the police, but they cannot find Mr. St. Clair. The room, in the window of which she saw her husband, is that of a dirty, disfigured beggar known to the police as Hugh Boone. The police are about to put this report down as a mistake of some kind when Mrs. St. Clair spots and identifies a box of wooden bricks that her husband said he would buy for their son. A further search turns up some of her husband's clothes. Later, his coat, with the pockets full of several pounds' worth of pennies and halfpennies, is found in the Thames, just below the building.
The beggar is arrested and locked up at the police station, and Holmes initially is quite convinced that Mr. St. Clair has been the unfortunate victim of murder. However, several days after Mr. St. Clair's disappearance, his wife receives a letter in his own writing. The arrival of this letter forces Holmes to reconsider his conclusions, leading him eventually to an extraordinary solution. Taking a bath sponge to the police station in a Gladstone bag, Holmes washes Boone's still-dirty face, causing his face to be revealed — the face of Neville St. Clair. Upon Mr. St. Clair's immediate confession, this solves the mystery, and creates a few problems.
It seems that Mr. St. Clair has been leading a double life, one of respectability, and the other as a beggar. In his youth, he had been an actor before becoming a newspaper reporter. In order to research an article, he had disguised himself as a beggar for a short time, during which he earned a very large amount of money. Later in his life, he returned to the street to beg for several days in order to pay a large debt. Given a choice between his newspaper salary and his high beggar earnings, he eventually became a professional beggar. His takings were large enough that he was able to establish himself as a country gentleman, marry well, and begin a respectable family. His wife never knew what he did for a living, and Holmes agrees to preserve Mr. St. Clair's secret as long as no more is heard of Hugh Boone.