At a funeral reception for John Truman, his grandson finds a statuette of a monkey covering its eyes, together with a diary from 1940. In it Truman describes how he lost his way while travelling to the remote village of Darton. Hearing a plane fly overhead, he dives for cover and cuts his hand. Truman spends the night in the car. The next morning he is brusquely woken by Ibbetson, leader of the Darton Home Guard.

Truman goes to the Darton doctor to bandage his hand. He is surprised when the doctor turns out to be a woman. He takes a room at the local pub, run by Joe and Gayle Hutton. Truman has been sent to the village by Reed of the Ministry of Information, to investigate an anonymous letter claiming that there are spies in Darton. Reed makes a reference to 'Rainy Day' without explaining its significance.

Truman is invited to a shooting party, where he learns that many in the village distrust Alice Durkow, the widow of a German internee who now billets Joan, Susan and Linda, the land girls working on Ibbetson's farm.

Dr Miller treats Joe for insomnia, but realises that he is also suffering from impotence. He is crushed that she can't help him, but refuses her suggestion to see a specialist. Ibbetson is infuriated by Joan when she fixes an engine he wasn't able to start. Linda spies Alice and Joan in their undergarments using some electrical equipment. That night Truman suffers from bad dreams after hearing Joe and Gayle argue about the rumours of an affair between Alice and Joan. The next morning Gayle has severe bruising to her face. Alice berates Truman for not having stopped Joe from beating her. She takes Gayle to her house while Truman fetches the doctor. Truman later shows Alice the anonymous letter and she admits that her son must have written it. Truman is made uneasy by her sensuality and directness.

Ibbetson is friendly with Linda and she tells him of Alice's electrical equipment. He convinces Truman to lead the Home Guard in a raid on Alice's house. Truman is outraged when he sees the men ransack the house and restrains Ibbetson after the latter strikes Joan. The machine is found, but proves only to be an electrical hair-removal device. Truman is disgusted by Ibbetson and promises to report him. He and the doctor go to her house for a drink. He asks her if she thinks there is any truth to the rumours about Alice and Joan. She thinks there might be, but that it shouldn't matter. He picks up her statuette of a monkey covering its eyes.

That night reports come in that the German have landed. Truman is told to come to Alice's house, where Joe has been found dead and sexually mutilated. Ibbetson refuses to contact the authorities and spitefully immobilises Truman's car. Alice and the girls are found by Truman and are hidden in the doctor's house. They claim Joe sexually assaulted them and that the gun went off during the struggle. Alice suggests that he go to the nearby military base, and accuses him of being afraid when he instead chooses to stay with them. Truman changes his mind and eventually makes it to the base. He is refused help, however, as the base is on full alert. Truman is arrested when he tries to steal a vehicle.

Reed arrives the next morning and takes him to the doctor's house, which has apparently been destroyed by a German bomber. She and the other women are dead. Truman finds the doctor's statuette as well as a detonating fuse. Reed admits that he is involved in secret activities and that 'Rainy Day' is a code name for one of them. In reality, the women were tortured and murdered by the Home Guard, but Reed has covered it up for the sake of morale. The invasion report was actually a false alarm. Reed lets Truman rejoin his regiment on condition that he remain silent.

Truman's grandson puts the diary away and returns to the house. His grandfather became a Colonel and had a distinguished service record.