An outbreak of apparent kleptomania at a student hostel is not normally the sort of crime that arouses Hercule Poirot's interest. But when he sees the bizarre list of stolen and vandalized items - including a stethoscope, some lightbulbs, some old flannel trousers, a box of chocolates, a slashed rucksack, some boracic powder and a diamond ring later found in a bowl of a soup - he congratulates the warden, Mrs Hubbard, on a 'unique and beautiful problem'. It is nevertheless not long before the crime of theft is the least of Poirot’s concerns.
Poirot’s solution of the petty thefts is unsubtle but effective: once he has threatened to call in the police, Celia Austin quickly confesses to the pettier amongst the incidents. She denies specifically: stealing Nigel Chapman’s green ink and using it to deface Elizabeth Johnston’s work; taking the stethoscope, the light bulbs and boracic powder; and cutting up and concealing a rucksack.
Celia appears to have committed the lesser thefts in order to attract the attention of Colin McNabb, a psychology student who at first regards her as an interesting case study, and then – almost immediately – becomes engaged to her. Celia makes restitution for the crimes and is seemingly reconciled with her victims, but when she is discovered the following morning dead from an overdose of morphine it does not take the investigators long to see through attempts to make her death seem like suicide.
Several of the original incidents have not been solved by Celia’s confession. Inspector Sharpe quickly solves the mystery of the stolen stethoscope during his interviews with the inhabitants of the hostel. Nigel Chapman admits to having stolen the stethoscope in order to pose as a doctor and steal the morphine tartrate from the hospital dispensary as part of a bet to acquire three deadly poisons. He claims that these poisons were then carefully disposed of, but cannot be sure that the morphine was not stolen from him while it was in his possession.
Poirot turns his attention to the reappearance of the diamond ring, and confronts Valerie Hobhouse, in whose soup the ring was found. It seems that the diamond had been replaced with a zircon and, given the fact that it was difficult for anyone but Valerie to have put the ring into the soup, Poirot accuses her of having stolen the diamond. She admits to having done so, saying that she needed the money to pay off gambling debts. She also admits to having planted in Celia’s mind the entire idea of the thefts.
Mrs. Nicoletis has been behaving very nervously, as if she were losing her nerve. One night someone gets her drunk and kills her.
Poirot focuses his attention now on the cutting up of the rucksack. By comparing an example of the rucksack type destroyed with others, he identifies an unusual corrugated base, and suggests to the police that the rucksack may have been part of a clever international smuggling operation. The rucksacks were sold to innocent students, and then exchanged as a means of transporting drugs and gems. Mrs. Nicoletis had been bankrolling the organisation, but was not the brain behind it. When the police visited Hickory Road on an unconnected issue, the murderer had cut up the rucksack to avoid its being found and removed light bulbs to avoid being recognised.
Patricia Lane comes to Nigel and admits that, in an effort to keep a dangerous poison safe, she has taken the morphine from the bottle in his drawer and substituted for it bicarbonate of soda. Now, however, the bottle of bicarbonate of soda has been taken from her own drawer. While they are searching for this bottle Patricia mentions that she is intending to write to his father in order to reconcile the two. Nigel tells her that the reason for his estrangement from is father is that he discovered that his father had poisoned his mother. This is why he changed his name and carries two passports.
Nigel comes to Inspector Sharpe and tells him about the missing morphine, but while he is there, Patricia telephones to say that she has discovered something further. By the time that Nigel and Sharpe get to the house, Patricia has been killed by a blow to the head. Mr. Akibombo comes to Sharpe and says that he had taken Patricia’s bicarbonate to ease a stomach complaint; when he took a teaspoonful of the bicarbonate, however, he had stomach pains and later discovered that the white powder was in fact the boracic powder. By the time Patricia had substituted the bicarbonate, the morphine had already been substituted by the stolen boracic powder.
Poirot, whose suspicions about Valerie Hobhouse’s role in the smuggling operation have been proved correct by a police raid on her beauty shop, now closes the case. The murderer has been the most obvious person, Nigel Chapman, who was known to have the morphine in his possession. He killed Celia because she knew about his dual identity and also knew that Valerie travelled abroad on a false passport. He killed Mrs. Nicoletis because she was sure to give the smuggling operation away under pressure, and killed Patricia because she was likely to draw to his father’s attention the recent events.
When Poirot outlines to Nigel’s father’s solicitor the case against Nigel, the solicitor is able to provide final proof. Nigel’s mother had been poisoned, not by his father, but by Nigel himself. When the father discovered this he forced him to write a confession and left it with his solicitor together with a letter explaining that it should be produced were there any evidence of further wrongdoing by his son.
Valerie confirms Poirot’s solution further. She has placed the call to the police station, apparently from Patricia, after Nigel had already killed her. The green ink was a double-bluff intended to divert suspicion away from him. Valerie is willing to incriminate Nigel fully because Mrs. Nicoletis was actually her mother.