At the age of eighty-seven, a millionaire, Greek by birth, Aristide Leonidis dies. His attending physician refuses to issue a death certificate and insists on an autopsy. His granddaughter Sophia asks her fiancé Charles Hayward, the son of an assistant commissioner of Scotland Yard, to help her in the investigation.

Charles discovers that Leonidis came to England and opened a chain of restaurants. Being an adroit and enterprising person, never breaking the law, he made a huge fortune. The small and ugly Leonidis always attracted women. He built a huge, ridiculous house and settled there with his family. Now there lives his son Roger with his wife Clementine; the second son is the writer Philip with his wife Magda, a famous actress, and three children, Sofia, Eustace and Josephine; Miss Haviland, the unmarried sister of his first wife, and his second wife, thirty-four-year-old Brenda, whom the millionaire married ten years ago. Leonidis had diabetes and was given insulin injections. Once instead of insulin, eye drops were injected, which was the cause of death. Brenda gave the injection, but many people live in the house, and everyone could replace the medicine. The bubble from which the fingerprints were erased was found in the trash bin.

Charles, along with a police inspector, arrives at Sofia’s house. Philip believes that his father mixed up the bubbles. Nobody benefited from his death, since he had provided enough for everyone during his lifetime. Although the family did not approve of his second marriage, the deceased had a good relationship with his wife. Roger blames his father's death for Brenda, who denies any guilt.

The inspector talks to the tutor of the youngest children, Philip Lawrence Brown, according to rumors that are in a love relationship with Brenda. But the young man throws a tantrum due to the fact that he is accused of murder. Brenda sympathizes with Lawrence, who is not easy to deal with with children. Sixteen-year-old Eustace, who had suffered polio, makes fun of him all the time, while twelve-year-old Josephine watches over everyone and overhears everything.

Charles meets Josephine, an extremely ugly but smart girl. She likes detective stories, and she decides to play detective. Josephine tells Charles everything that happens in the family. Philip wrote an unsuccessful play, in which the main role was intended for Magda, but Leonidis refused to finance it. She herself wanted to study for a ballerina, but grandfather was against it. Roger and Clementine want to leave. Before his death, Roger spoke about something with his father for a long time. According to Josephine, Roger pocketed his father money. Lawrence and Brenda write sentimental letters to each other.

Leonidis' lawyer, Mr. Gateskill, announces that a will has disappeared in which the deceased left a decent amount to each family member. Now the state should go to Brenda.

Charles, along with his father, who helps his son in the investigation, calls Roger to the police. Roger admits that he has bankrupted a company that his father entrusted to him. He wanted to escape, his father called him and offered help, but now, in connection with his death, he will not receive help. Charles excludes Roger from among suspects.

Josephine tells Charles the news. Miss Haviland wants to help Roger, but father and mother expect to stage a new play. With a mysterious look, Josephine announces that she almost knows who the killer is, but will remain silent until the end.

Sofia worries that she lives in a house with a killer. She is also worried about Josephine, who overhears everything. Once she asked grandfather why “external” is written on a bubble with eye drops, and grandfather explained that this medicine should not be taken inside. If Brenda mixes up the bubbles, he will die.If everyone heard this, then why would Brenda kill her husband, because in a family where everyone does not love her, suspicion will fall on her.

Sofia is also worried about Eustace. He hates everyone after an illness. Her parents do not pay attention to the children: the father is busy with his work, and the mother is amused by the fact that she tetralizes life, making theater scenes from everything. Now, looking at Miss Haviland, she dreams of playing a woman who is in love with her sister's husband and hates his second wife.

Magda decides to send Josephine to a Swiss school. Her father-in-law loved that the whole family was together, but Magda believes that the girl needs to communicate with her peers. Charles begins to suspect Magda of killing his father-in-law and wanting to send the curious daughter away.

Josephine is looking for something in the closet and reports that the time has come for a second murder. From Eustace, Charles learns that Josephine writes down all her observations in a diary, which she carefully hides.

A friend of Leonidis gives Mr. Gateskill a testament written a year ago. The lawyer sees that this is another document. The deceased left his wife a huge amount of money, and the rest of the property passed to Sofia. A letter is attached to the will in which Leonidis explains that he sees only the granddaughter as a worthy continuer of his work. The old sly man arranged everything with the loss of his will.

Sofia urgently asks Charles to come - an attempt was made on Josephine, and the girl was taken to the hospital. The girl loved to ride with her foot in the hole on the laundry door. Someone placed a marble pillar on top of the door that hit Josephine in the head. A chair with pieces of fresh earth stood next to the door - they stood on it to lay a pillar. Charles comes into the girl’s room to find the diary, but there everything is turned upside down. Who could do this is unknown, since due to the chaotic layout of the house it is impossible to trace the movements of residents.

Charles rushes into the closet, where Josephine was on the eve. There he finds the hidden letters of Lawrence and Brenda.

Family members are outraged by the will, and Eustace does not hide his hatred of his grandfather.

Police arrest Lawrence and Brenda, although they deny their guilt.

Josephine is returning home. She knew about the will, since she overheard everything. Miss Haviland asks Charles to take her to London and does not tell anyone about it. In London, she must go to the doctor.

Charles's father doubts that Lawrence and Brenda are murderers, he believes that the criminal is someone from the family. Most likely it was Eustace, and it was he who was looking for something in Josephine’s room, but not a letter, but a diary.

Sophia again asks Charles to come - the nanny Josephine has died. She drank the girl’s cocoa, and there appeared poison, Miss Heviland’s heart drops. Charles asks the girl to say who the killer is, but she refuses. Miss Haviland offers the girl to go for a walk with her, promising various entertainments.

Police come to the house. Miss Heviland and Josephine crashed in a car. Charles recalls that Miss Haviland wrote letters before the trip. One of them is addressed to the inspector, in which she confesses to the killings. Miss Haviland also reports that she is terminally ill and has several months left to live. The second letter is addressed to Charles. There was Josephine's diary in which the girl wrote down how she committed the killings. Miss Haviland accidentally found him and decided that the child should not be hurt for the perfect.

Grandfather did not allow Josephine to be a ballerina, and she decided to kill him, especially since he himself told her how. The nanny chuckled at her, and she killed her, but did not have time to decide who would put the empty vial. The girl planned to kill her mother so that she would not send her to Switzerland, and Eustace for laughing at her. She planned to hand over her little book to the police in her old age so that everyone would know how great she was.

Charles now realizes that Josephine herself set up the attempt. Only she had to stand on a chair to put a backup on the door.Charles offers Sofia to marry him and leave so that everything is forgotten. Grandfather was not mistaken, considering that Sofia is an honest and courageous person.

Charles's father also guessed who the killer was. He is sorry for the unfortunate girl.