The ambassador at a foreign embassy in London goes home to collect his wife from hospital. She has been recovering from an illness for the last eight months. Their son Felipe stays behind, under the supervision of Baines, the butler, and his wife. After waving goodbye to his father, Felipe goes to retrieve his pet snake, McGregor, from a loose brick outside his bedroom window. He hides the snake from Mrs Baines, who wants him to get rid of it. Felipe goes to the kitchen and asks Baines to take him out for a walk in the afternoon. However, Mrs Baines won't let him go, and Felipe, in anger, says that he hates her.

That afternoon, Felipe sees Baines leave the house and follows him. He leaves by the fire escape and finds Baines in a teashop with Julie, a typist in the embassy. The two are clearly upset, but for Felipe's sake pretend to be talking about another couple's love affair and not their own. Baines tries to convince Julie not to leave him, promising to speak to his wife and seek a divorce.

Baines asks Felipe to keep their meeting with Julie a secret from his wife. Felipe is seen by Mrs Baines as he returns McGregor to his hiding place. She sends him to the kitchen, then puts the snake in a handkerchief and throws it in the furnace. Baines asks his wife for a divorce, but she angrily refuses. Later that evening, Mrs Baines scolds Felipe for eating cake during the afternoon, shaking him violently. Felipe inadvertently lets slip that there were three of them out in the park in the afternoon. She makes Felipe promise not to tell her husband that she now knows about their meeting. She later informs Baines that she plans to take the next day off to see her aunt.

Baines makes plans to meet Julie later in the day, unaware that his wife is hiding in the house. He takes Felipe to the zoo and entertains him with tall tales of his life in Africa, when he had to kill natives. They meet Julie and then return to the embassy, where Felipe takes her to see McGregor. Finding the snake gone, he is greatly upset when he realises that Mrs Baines must have got rid of it. They spend the rest of the evening playing, including a game of hide and seek.

Felipe is awoken later that night by Mrs Baines, who is clearly unhinged. Demanding to know where Julie and her husband are, she attacks the boy before being confronted by her husband. Hysterical, she demands to see Julie. Baines goes to get changed, and Mrs Baines tries to spy on him, standing behind a large swing window on a ledge over the main staircase. Trying to get a better view, she slips and falls to her death. Having seen Baines and his wife struggle at the top of the stairs, Felipe flees in horror when moments later he sees her body. He is eventually found by a policeman and taken to the station where he is comforted by a prostitute.

Felipe is returned to the embassy. Baines tries to keep Julie out of the story, claiming that his wife spent the evening with them instead. The next day, officers from Scotland Yard come to interrogate Baines and find his story less and less credible. Felipe tries to help, but ends up drawing attention to the contradictions in Baines' story. Julie eventually insists that the truth be told. However, the police remain unconvinced, as the angle of the body is inconsistent with a fall from the top of the stairs. Just when Felipe decides to tell the truth, the police discover Mrs Baines' footprints near the swing window. They realise that she must have fallen from there and that her death was indeed accidental. Felipe is confused that no one seems to want to listen to him now that he has decided to finally tell what really happened. The police depart shortly before Felipe's mother returns. Uneasily, he walks down the stairs to greet her.