The action takes place in post-war Italy, in a small Sicilian town. Pharmacist Manno receives an anonymous letter, where he is threatened with death, without going into an explanation of the reasons. Friends of the pharmacist - Don Luigi Corvaya, notary Pecorilla, teacher Laurana, attorney Rosello, Dr. Rosho - consider the anonymous letter a bad joke. Manno himself is inclined to think that they want to intimidate him in order to discourage him from hunting - in a few days the season opens, and envious people, as always, get numb. However, just in case, the pharmacist notifies the sergeant of the Carabinieri about what happened, and when he unfolds the letter, Paolo Laurana sees the word “Unicuique” on the back of the sheet, typed in a typical typographic font.

On the twenty third day of August 1964, on the opening day of the hunting season, pharmacist Manno and his regular partner Dr. Rosho are found dead. The author of the anonymous letter carried out his threat, and the inhabitants of the town begin to wonder what the late pharmacist did. All pity the poor doctor, who suffered for other people's sins. The police also zealously set to work: both victims occupied a prominent position and enjoyed general respect. In addition, Dr. Rosho has an influential relative: he himself is the son of a well-known oculist professor, and his wife is the niece of the canon and cousin of attorney Rosello.

With a united effort, the police and residents of the city find the solution to the murder: the pharmacist was clearly cheating on his ugly withered wife, and some jealous man killed him. The lack of evidence and the excellent reputation of the deceased do not bother anyone: since it came to the murder, it means that the matter is unclean. Only Laurana holds a different point of view: although the Sicilian instinct calls for caution, he finds out in a roundabout way that the Catholic newspaper Osservator Romano is written out by only two people - a canon and a parish priest.

The priest’s rooms for the last month are safe and sound. Laurana looks fascinated at the subtitle “Unicuique suum” (lat. “To each his own”). At the canon, a failure awaits him: in this house, the read newspapers become a household item. The canon is firmly convinced that the pharmacist paid for a love affair, and the husband of his beloved niece simply turned up the killer's arm.

The investigation could have ended on this, but, unfortunately, Laurana was lucky. This quiet and shy teacher of Italian in the town is respected, but he has no close friends. He was associated with Dr. Rosho's school memories - they studied together at the gymnasium and at the lyceum. After death, Rosho Laurana experiences a feeling of emptiness and pain - this was almost the only person with whom he could discuss literary novelties or political events. Laurana’s personal life did not work out because of a selfish and jealous mother - on the threshold of her fortieth birthday, he remains for her a naive and inexperienced boy, not matured for marriage.

In September, Laurana arrives in Palermo to take exams at the Lyceum. In the restaurant, he meets a former schoolmate - now a member of parliament from the Communist Party. Rosho voted for the Communists, although he hid it out of respect for his wife's relatives. Shortly before his death, the doctor visited Rome to meet with the deputy and find out whether it is possible to put revealing materials in the newspaper about one of the most eminent citizens of the town, who is holding the entire province in his hands and is involved in many dirty cases.

Returning home, Laurana talks about his discovery to lawyer Rosello. He is eager to take revenge on the unknown killer. The beautiful widow of the doctor is also thrilled, because before she sincerely believed that her husband had died due to the pharmacist's love affairs. Signora Louise even allows Laurana to look at the papers of the deceased, although she is extremely saddened by the version that the pharmacist served as a false bait - everyone in the town knew that Manno and Rosho were hunting together.

Laurana turns to the parish priest for help, to whom she is sympathetic, despite her anti-clerical beliefs. He says that the most influential person in the province is the lawyer Rosello, who has reached a high position by bribery, bribes and other frauds. Laurana's eyes suddenly open: in the town it has been rumored for a long time that the lawyer and his cousin love each other from a young age, but the canon opposed the marriage between close relatives, so Louise married Dr. Rosho. The beauty of this woman immediately aroused an acute desire in Laurana, and now horror has been added to this feeling - undoubtedly, she was an accomplice to a cruel and insidious crime.

—The fatal incident once again comes to the rescue of Laurana. Thinking of getting a driver’s license, he goes to the Palace of Justice and encounters Rosello’s lawyer, who is going downstairs in the company of two men. Laurana knows well the deputy Abello, famous for his scholarship, but he sees his companion for the first time. This man with a broad, rude face smokes Branca cigars - at the scene of the murder of pharmacist Manco and Dr. Rosho, a cigarette butt of such a cigar was found. Soon, Laurana finds out that he was not mistaken in his assumptions: the man who smoked cigars is a member of the local mafia.

After meeting at the Palais de Justice, attorney Rosello begins to avoid Laurana. On the contrary, the beautiful Signora Louise shows a keen interest in him. Laurana almost pities Rosello and is not going to report it: he has a deep aversion to the law and, like all Sicilians, at heart considers the double-barreled shotgun the best way to fight for justice. In early November, Laurana goes to class and on a regular bus, she is surprised to notice the widow of Rosho. Signora Louise admits that she thought a lot about her husband’s trip to Rome, and recently she managed to find the doctor’s secret diary behind books. Now she had no doubt: the murder most likely was set up by cousin Rosello. Laurana does not believe her ears: this lovely woman is pure - in vain he offended her with suspicions. They make an appointment at Romero Cafe at seven in the evening. Laurana waits until half past nine in excitement - there is no Louise, and anxiety for her life is growing in him. He goes to the forecourt, and then a resident of the town, who is familiar to him in person, but not by name, kindly offers to give him a lift.

The case of Paolo Laurana’s disappearance has to be closed: he was seen in the Romeris cafe, and he was clearly waiting for someone - apparently, it was a love date. Perhaps he will return home, like a walking March cat. The police do not know that Laurana’s body lies at the bottom of an abandoned sulfur mine.

A year later, on the day of the feast of Mary the Lady, canon Rosello, as usual, gathers friends. The mourning is over, and you can announce the engagement of the nephew of the lawyer with niece Louise. Notary Pecorilla and don Luigi Corvaya overlook the balcony. Both are eager to share the innermost: the poor pharmacist had nothing to do with it - Rosho found his wife and cousin at the crime scene and demanded that Rosello get out of town, otherwise the press would reveal information about his dirty deeds. As for the unfortunate Laurana, he was just a fool.