: They are trying to steal a diadem from a banker as collateral and break off one tooth. He is found by Sherlock Holmes, who finds out that the abduction was organized by a fraudster who seduced the banker's niece.

Sherlock Holmes is contacted by banker Alexander Holder. One English senior person who urgently needed money left him a pledge of a beryl diadem, a national treasure. The cost of the jewel is at least twice the loan. The diadem contains thirteen teeth and thirty-nine large beryls, three on each tooth.

Fearing to keep such a thing in a bank safe, Holder brought it home and locked it in a bureau in the room adjacent to his bedroom. There are four maids in the banker's house. Three have been working for a long time, and their honesty is beyond doubt. The fourth, Lucy, is working recently. She acted with excellent recommendations and copes well with her duties. Pretty Lucy has a lot of fans, but otherwise she’s a pretty decent girl. Holder himself is a widower and has an only son, Arthur. The young man did not live up to his father’s hopes; he is wasting money on cards and at the races. He tried several times to get out of this circle, but his friend, Sir George Burnwell, has a bad influence on Arthur and each time returns him to the same path.Also in the house is the niece of Holder, Mary. After the death of her father, the girl was left completely alone, and Holder took her to him. Mary is Holder's right hand, the sunlight in the house. Arthur is hopelessly in love with her, he already twice asked Mary to marry him, but she refused. Mary, as well as Holder, does not approve of Arthur’s friendship with Sir George, believing that this person can not be trusted.

After lunch, when Lucy had already left the room, Holder told Arthur and Mary about the diadem. Arthur was surprised that her father put her in the bureau, believing that it could be easily opened. He asked his father for money, but was refused.

Before going to bed, Holder walked around the house. He saw Mary closing the window in the living room. Mary complained about Lucy, to whom a fan came. Holder almost awake at night heard footsteps in the next room. He rushed there and saw a barefoot Arthur in trousers and a shirt. Arthur was holding a diadem in which one tooth with three stones was missing. He denied the theft, but did not explain why he ended up in the room at night. Running to the noise, Mary fainted. Holder called the police. They searched the whole house, but they did not find the missing tooth. Arthur was arrested, and Holder decided to seek help from the great detective.

After hearing the story, Holmes is inclined to think that Arthur is innocent, otherwise he would have come up with an excuse, and not be silent. And if Arthur broke the tooth, then where did he hide it so that no one can find it?

Holmes arrives at the banker's house. Deadly pale Mary believes that Arthur is innocent. She assumes that Lucy is to blame, who heard a conversation about a diadem and, together with her greengrocer, committed theft.

Holmes examines the diadem. If Arthur broke a tooth in the house, then a terrible crack would sound.Having examined the area around the house, Holmes invites Holder to stop by tomorrow. The rest of the day, Holmes disappears somewhere, dressed as a tramp.

The next day, Baker arrives at Baker Street. The banker has a new misfortune. Mary disappeared at night. The note left by her says that she caused uncle a lot of grief and cannot stay in his house.

Holmes returns the missing tooth to the banker for a fee and demands to apologize to his son for the charge of theft. Theft was committed by Mary for Sir George Burnwell, with whom she was in love. Coming to the House of Holders, Sir George, one of the most dangerous subjects, a player, a notorious villain, fascinated the girl, they saw each other almost every day. That evening, Mary told Sir George about the diadem, and he persuaded her to steal the jewel. Uncle Mary said this was a fan of Lucy. Arthur, who was worried about thoughts of debt, did not sleep that night and heard footsteps. Looking out into the corridor, he saw Mary with a diadem in his hands. Hiding, he watched Mary give someone the jewel through the window. Arthur rushed into the street and caught up with Burnwell. A fight broke out between them, during which a tooth broke off. At the moment when Arthur wanted to put the diadem in place, his father entered. Afraid of exposing his beloved girl, Arthur was silent, but he also did not want to take the blame on himself.

Examining the ground near the house, Holmes found traces of Arthur's bare feet, drops of blood and traces of someone’s shoes. It is unlikely that Arthur would protect the maids, but he did not become extradited to Mary. Knowing that Mary rarely happens in society, and Sir Burnwell often comes into the house, the great detective suggested that it was he who knocked Mary to theft.Having disguised himself as a vagabond, Holmes came to Burnwell's house and learned from the servants that on the eve of their master broke his head. Taking his shoe, Holmes made sure that it was his footprints near the banker's house. Having promised not to institute criminal proceedings, Holmes took the missing tooth.