Tommy sorts out a pile of books in the office. They are a number of volumes of famous detective stories and he thinks it would be a good idea to base their techniques on the different styles of their fictional counterparts. He has also bought a good camera for taking photographs of footprints and "all that sort of thing".
They receive a client. It is a young woman named Miss Kingston Bruce. She lives in Wimbledon with her parents and last night one of their guests lost a valuable pink pearl. They do not wish to call in the police yet and the Blunts were recommended to them by Lawrence St. Vincent who was also one of the guests.
The Beresfords travel to Wimbledon and meet Colonel Kingston Bruce. He proudly tells them that Lady Laura Barton, daughter of the late Earl of Carroway, is staying with them together with an American couple, Mr. and Mrs. Hamilton Betts, who wanted to meet a titled lady. During a bridge game after dinner, the clasp of Mrs. Betts' pearl pendant necklace broke. She laid it down on a small table and forgot to take it upstairs. The next morning, the necklace was still there but the pearl itself had gone. Aside from the Kingston Bruce's, the Betts, Lady Laura and St. Vincent, the only other guest was a Mr. Rennie who is paying court to Miss Kingston Bruce. Her father doesn't like or trust the man as he is a socialist. No one has been allowed to leave the house since the pearl was discovered to be missing except for their daughter when she went to the Blunts. They are given approval to search the house as part of their investigations.
Tommy tries to look impressive by using his new camera whilst Tuppence tactfully questions the servants. They overhear a scrap of conversation between Mrs. Kingston Bruce and her daughter about someone hiding a teaspoon in their muff and wonder who this can be. Later on, Tuppence ferrets out of Lady Laura's French maid, Elise, that her employer is something of a kleptomaniac and five times in the past items have gone missing when she had been staying at friend's houses. They start searching Lady Laura's bedroom and bathroom, momentarily getting stuck in the latter room when Elise cannot open the door. Tommy takes pictures in the bedroom with Elise's assistance and then quietly tells Tuppence that he has an idea and has to go out to pursue it. In the meantime, she is not to let Lady Laura out of the house.
Some time later Tommy returns with Inspector Marriot of Scotland Yard. They go straight back to the bathroom and cut the cake of soap in half. Inside it is the pearl. The reason Elise couldn't open the door was that she had soap on her hands after depositing the pearl there. Tommy's photographs included one of the maid and she handled one of the glass slides, thus leaving her fingerprints. The Yard have identified her from their records as a missing criminal and arrested her. Being the maid of a lady suspected of kleptomania was the best cover she could have.