Act one. Suspicion

Sir Charles Cartwright, a middle-aged man, a famous actor who recently left the stage, gathers guests in his villa. He is not a superstitious person, and the fact that 13 people should get together, “a damn dozen,” does not bother him. Among those invited are his old friend Dr. Bartholomew Strange, a specialist in nervous diseases, pastor Bebbington with his wife, Lady Mary Lytton Gore with her daughter Egg, whom Sir Charles is not indifferent to, another friend Mr. Satterthwaite and the famous detective Hercule Poirot.

Enthusiastic conversation guests Sir Charles offers a cocktail. Pastor Bebbington drinks a drink and falls dead. Those present were shocked: the pastor, although he was an old man, did not suffer from any illness from which he could suddenly die, he also had no reason to commit suicide. He was a sweet, harmless old man, no one could need to kill him. And how could one pour poison into his glass in front of everyone?

In the meantime, Lady Mary consults Mr. Satterthwaite about her daughter's future. Egg is looked after by a very nice young man from a wealthy family who has a good job, but Egg is not indifferent to Sir Charles. Lady Mary does not object to such a marriage, despite the fact that Sir Charles is almost twice older than Egg. Yes, Sir Chals has a womanizer reputation, but that’s all in the past. In turn, Sir Charles doubts the girl’s feelings, is jealous of her young admirer and decides to go abroad.

Act two. Confidence

Mr. Satterthwaite is leaving for Monte Carlo. There, he caught his eye in a newspaper article with the news of the death of Dr. Bartholomew Strange. The deceased had a dinner party with the same guests that Sir Charles had, except for Mr. Satterthwaite, Sir Charles, and Hercule Poirot. He suddenly died in the midst of a lively conversation. Mr. Satterthwight recalls the strange death of Pastor Bebbington.

In Monte Carlo, Mr. Satterthwaite meets Sir Charles and finds out that the pastor and the doctor were poisoned with nicotine, but no poison was found in the glasses of the dead. Friends decide to return to England, especially since Sir Charles received a letter from Egg. In England, they learn the details of the tragedy. Shortly before his death, Dr. Strange hired a new butler Ellis. Arriving at the scene, police interrogated him along with the others. The butler was courteous during the interrogation, satisfactorily answered all the questions, and after the interrogation he disappeared, most likely he went through a secret passage.

The police are busy with the strange disappearance of the butler. Nothing was lost in the house, nothing suspicious was found in the doctor’s personal papers.

Mr. Satterthwaite and Sir Charles are discussing what happened. They conclude that both killings are interconnected, but they cannot figure out how. They discuss everyone who was present at the banquets, but they cannot find reasons for the murder. Friends decide to talk to the servant. One of the maidservants notes that the owner behaved differently with the new butler than with the other servants, and the butler himself “did everything like what was supposed to, but somehow not like the others.” She also remembered the call from the doctor’s clinic - they reported that a new patient had arrived.

Friends go to the clinic to find out about the new patient. But the woman is in poor condition and they are not allowed to visit her. They inspect the butler’s room. Sir Charles finds draft letters from which he learns that the butler is Ellis. in need of money, knows why Dr. Strange died.

Act Three. Exposure

Sir Charles, Sutterswright, and Egg, who joined them, are trying to sort things out. Hercule Poirot suddenly appears and offers his help. If there could be reasons for the doctor’s murder, then no one has benefited from the death of Pastor Bebbington.

Sir Charles, Sutterswright and Egg once again interrogate all those present at the tragedy. After listening to them, the party dinner is arranged by Hercule Poirot. He invites all witnesses to the killings. While the famous detective entertains the guests with chatter, Sir Charles falls dead. The guests are stunned, but Sir Charles jumps up and bows. Poirot explains the essence of the play: when everyone rushed to the "corpse", the killer quietly changed the victim's glass, so poison was not found in the glass of the dead. The detective asks those present not to hide anything from him. Disappointed guests disperse.

Poirot receives a letter from a patient of Dr. Strange with a request to visit her - she wants to provide important information. He goes to the clinic, where he finds out that the patient died, poisoned by mail received sweets, which contained nicotine.

Sir Charles and Egg decide to visit the secretary's mother, Sir Charles, who lives next door to the Bebbington family, in the hope that she knows something about the pastor. During the trip, Sir Charles confesses to Egg that his real name is “Bidon”, and he took a more sonorous one for the stage. Sir Charles and Egg confess each other their love. They notice that the secretary of Sir Charles strangely received the news of the poisoning of victims by nicotine.

After listening to Egg's information, Poirot decides to follow the secretary and catches her at the moment when she tries to break the instruments in the secret room of Sir Charles's estate.

Poirot again gathers guests to name the criminal. Upon reflection, the famous detective concluded that all three murders were committed by one person. The first two occurred on the same principle, which means the killer who was present at both dinners. Given Sir Charles’s acting background and the strange butler’s disappearance, Poirot decides that Sir Charles played the role of the butler with the help of Dr. Strange. He poisoned the pastor and the doctor. In order to avert suspicion from himself, he sends a telegram to Poirot on behalf of the patient, Dr. Strange, and then sends her poisoned sweets. But he makes a mistake, because the woman could not know that the famous detective is engaged in this investigation. Then Poirot guessed who the killer was.

But why did Sir Charles do this? Being passionately in love with a girl who reciprocates with him, Sir Charles does not make her an offer, since he is married to a woman who is in a psychiatric clinic and cannot divorce her. Dr. Strange was an old friend of Sir Charles and knew this secret. He could open it to prevent imaginary marriage with Egg. To carry out the murder, the actor needs a rehearsal, and he kills one of the guests in order to rehearse the substitution of glasses. The secretary guessed that her master was a killer, and decided to destroy his laboratory.

Mr. Sutterswright in horror understands that the pastor was killed by accident, to which Poirot replies that the case could turn out even worse: Hercule Poirot himself could have been at the victim's place.