Through the fault of the young man, a document of national importance disappears. Sherlock Holmes finds out that the document was stolen by the bride's brother of a young man.

Sherlock Holmes seeks help from Percy Phelps, Dr. Watson's school friend. Phelps is sick and cannot visit an apartment on Baker Street, so Sherlock Holmes and Dr. Watson go to the estate near London. They are met by Mr. Joseph Harrison, brother of the bride of Percy, whose room he now occupies.

Percy's mother's brother, Lord Holdhurst, an influential politician, arranged for Percy to work for the Foreign Office. Percy turned out to be a good worker, and they began to give him responsible assignments. Recently, his uncle showed him an important treaty between England and Italy. Despite the strictest secrecy, information about it has already leaked to the press, and now Russia and France are ready to pay huge sums of money to find out its contents. A copy must be taken from the document. Lord Holdhurst handed Percy a contract to lock him in a safe in his office. After work, when everyone left, Percy had to calmly take a copy, put the documents in a safe, and return them to his uncle in the morning. The conversation took place in private, and eavesdropping was excluded.

One of the employees, Charles Goro, was late at work, so Percy went to lunch. When he returned, Goro had already left, and Percy set to work. He was in a hurry to finish it, so that he could leave Joseph with the last train. Without even half, Percy felt tired. He called the attendant on duty to bring him coffee. The doorman's wife, who worked as a cleaner, came on the phone. After a while, seeing that coffee was not brought, Percy decided to find out what was the matter. He saw that the kettle was boiling with might and main, and the doorman was sound asleep. Percy woke him, and then a bell rang from Percy’s office. Throwing everything, Percy ran to his room. There was no one in the corridors or in the rooms, a copy lay on the table, and the original disappeared. Since there was nowhere to hide in the corridors and back streets, Percy decided that the kidnapper had come from the street, but the policeman standing there did not see anyone except the doorman's wife, who was going home after work. Percy wanted to run after her, but the doorman dissuaded him.

It was raining that evening, passers-by rushed home, and Percy was unable to figure out anything. Having examined the office, he saw that there were no traces on the floor, the windows were closed from the inside, therefore, the offender could only enter through the door. But how could he get unnoticed, and why did he ring the bell? Percy's suspicion fell on the doorman's wife, Mrs. Tanji. Together with the inspector of Scotland Yard, Percy went to her home. The frightened woman tried to hide, explaining that she owed money to the shopkeeper and thought that they had come to describe the property. Arresting her, the police searched the house of Tanji in the most thorough manner, but found nothing. Also, verification by Charles Goro, a Frenchman by birth, did not give anything.

Percy experienced serious illness. He started to have a nervous fever. For ten weeks, Annie, the bride of Percy, has been caring for him, not leaving the patient for a minute. The police came to a standstill, and Percy had one last hope - the great detective Sherlock Holmes.

Sherlock Holmes will also check the family of Tanji and Charles Goro, but he does not find any incriminating evidence. Holmes decides to check on Uncle Percy, Lord Holdhurst, but a conversation with him does not clarify the situation.

The next morning, Holmes and Dr. Watson visit Percy Phelps, who informs them that they tried to attack him at night. Feeling better, he decided to spend the night without a nurse. Suddenly he heard a rustle turning into a rattle: someone was trying to open a window from the outside. Opening the shutters, Percy saw a man wrapped in a cloak under a window with a knife in his hand. Noticing Percy, the man ran away. Due to his weakness, Percy could not rush after him, and only Joseph ran to the bell. Having examined the place under the window and the garden, Joseph saw traces that were lost in the grass, and a broken board in the fence. Holmes concludes that the board was broken long ago, there are no marks on the other side of the fence, and it is not noticeable that anyone would jump off it.

Holmes sends Percy and Dr. Watson to London, and asks his bride to stay and not leave the room for a day, and to go to bed, lock the room with a key and take it with him. The detective himself also remains in the estate.

In the morning, Sherlock Holmes appears on Baker Street with a bandaged hand. Without explaining anything, he invites friends to have breakfast. Picking up the lid from his plate, Phelps, instead of the promised scrambled eggs, finds a contract.

Burning with curiosity to friends, the great detective tells how, having led them, he climbed over the fence and invisibly to the inhabitants of the house penetrated the window of Percy's bedroom. Percy's bride, read late into his room, as Holmes had promised, and left, closing the door with a key and taking it with her. At night, opening the door with a key, Joseph entered the room, wrapped in a black cloak and holding a knife in his hand. Since he opened the window, Holmes saw everything that was happening in the room. Pulling the contract from the cache, the kidnapper jumped out the window and fell into the arms of the great detective.

After studying the facts, Holmes concluded that Joseph was well acquainted with the location of the rooms in the house and with the location of the rooms in the ministry. He went to work with Percy. Since there was no one in the room, he rang the bell and then noticed a document lying on the table. Realizing that the document was of great national importance, he put it in his pocket and left. Putting him in a cache in his room, Joseph hoped to take it a few days later and take it to the French embassy, but then his room was placed at the disposal of the sick Percy, who always had a nurse. When Percy got a little better, and he was left alone for the night, Joseph tried to get into the cache, but the attempt was unsuccessful. Upon learning that the room was empty, the thief decided to try again, but the great detective upset his plans.