The story opens with the narrator explaining the odd arrangement in the Brown household. Chief Brown is the head of the police, and by all accounts a capable one. But when a case defeats him, he does not sit at his desk brooding over it. He clears the desk, puts on his hat and goes home for dinner — because his son will have the answer before the meal is over.
The son's real name is Leroy, and only his parents and teachers use it. Everyone else calls him Encyclopedia, because he has read more books than anyone in his town and never forgets a fact. The narrator tells us his friends described him as "a library and computer rolled into one" — that is the friends' joke about him, not the narrator's own claim. The nickname is not something he chose or enjoys; the narrator says plainly that he was "stuck with it". His father would love to boast about him but stops himself, because nobody would believe that the finest detective around is a schoolboy in the eighth class.
One Tuesday night at dinner, Chief Brown stares at his cream-of-mushroom soup. His wife and son both know what that means: a case he cannot crack. He announces, without any drama at all, that a man named Tim Nolan died the day before.
Mrs. Brown recognises the name, and the parents begin filling in the history. Encyclopedia says nothing, because he knows perfectly well that his parents are talking the case through "for his benefit". So the background comes out as ordinary family conversation.
Five years earlier, two masked men had robbed a jewellery shop called the Diamond Mart on Sixth Avenue and escaped with jewellery worth a million dollars. Chief Brown had always believed that Nolan and a man named Daniel Davenport were the robbers, but he could never prove it. The two had met while both were in prison in South Carolina and became friendly over shared interests. Nolan was released first, settled in the town and started a small nursery growing palm trees — a business that, in Chief Brown's words, hardly earned him enough to live on. Davenport moved into Nolan's house just one week before the robbery.
During the hold-up, one gunman's mask slipped and a shop clerk thought she recognised Nolan — but she was not absolutely sure, and she later refused to give evidence in court. Nolan was questioned but never arrested. Davenport vanished immediately after the robbery and was never seen again. Not one piece of the stolen jewellery ever turned up. Chief Brown's hunch was that the two men had hidden it together and were waiting for the search to die down.
Now Nolan is dead. His heart had been weak for many years, and on the morning of his death a stroke struck him. Realising that the end had come, he used whatever strength was left to leave his will lying on the kitchen table. The will leaves everything he owned, including the palm-tree nursery, to Davenport. Chief Brown obtained a court order that morning, and Officers Lewis and Maloney went through the house. They just about took it apart and did not find one piece of the stolen jewellery.
The suspicious detail is not the will itself but a sheet from Nolan's desk calendar, clipped to it. Chief Brown had copied what was written on that sheet into his pocket notebook: Davenport's name and address, and below them four words — "Nom Utes Sweden Hurts".
Mrs. Brown cannot make sense of them and passes the notebook to her son. Encyclopedia leans back and closes his eyes, which is what he does when the thinking is hardest. After a few seconds he opens them and asks a single question: is there a young fir tree in Nolan's palm-tree nursery? His father, taken aback, says yes — one, on the south side of the house.
Nobody else can see how the four words say any such thing. Mrs. Brown, who had taught English and other subjects in a high school, offers the dictionary meanings instead: nom is a shortened form of a grammar term, Utes is the name of a Native American people, and Sweden is a country in northern Europe. Of the last word she can only say "Hurts is hurts". Then she honestly admits that she cannot work it out.
Encyclopedia reminds her that Davenport disappeared right after the hold-up, and gives his own reading of events: as his father had said, the two men must have hidden the stolen jewellery together, and probably only a week or so ago Nolan changed the hiding place. It is his mother who supplies the next step, asking whether Nolan had tried to tell Davenport by phone, failed to reach him, and so wrote the four words down as he was dying. Chief Brown nods at that, and says they will find Davenport now that they know his address. Mrs. Brown then completes the thought herself: once Davenport hears that the palm-tree nursery has been left to him, those four words will point him straight to the hiding place.
Encyclopedia agrees. He adds that the code is simple, especially because of the kind of paper it is written on, and points out that it would not make much sense to anyone who was not already looking for a hiding place. Pressed by his impatient mother to say where the jewellery is, he smiles and gives the line that closes the scene: "Why, under the fifth word".
The story then puts its challenge to the reader before explaining anything. The answer follows. The four words stand for days of the week taken in order, Monday to Thursday, with the letters d, a and y dropped and the remaining letters used to form new words. The unwritten fifth word is Fir, from Friday. The jewellery turned up in a twenty-gallon jug of earth, the very jug in which the young fir tree was growing — exactly as Encyclopedia had predicted.