Returning from holiday, John Gray complains that nothing exciting happened. Taking the luggage out of his car, he notices a camera on his back seat. To try and identify its owner, he decides to develop one of the negatives. He is shocked to discover that the image shows a man stabbing another. When he goes downstairs to answer the doorbell, a man enters the darkroom through a window and takes the camera and the developed negative. Hearing John returning, the man tries to make a hasty exit, but the window sash breaks and he is caught under it. After struggling, he is able to free himself before being caught. Realising what has happened, John develops the remaining negatives and makes prints from them. One of them shows a young woman standing outside a front door with an address partly visible.
John tracks down the woman, who identifies herself as Mary Elton. Her brother Ernest took the photograph, just before he went missing. John and Mary look at some of the other photos and are able to identify part of a train line. While John makes a phone call, a policeman looking for her brother visits her. He tells her that Ernest is wanted in connection with a robbery at the jeweller's where he works. After he leaves, John and Mary drive out to the place where the photo of the train line was taken, not realising that they are being followed. They ask a local farmer if he recognises the other places showed in the photos, but he says that they can't be within a radius of fifty miles. They realise that the photo was taken from a moving train and by tracking the line manage to find an inn he photographed. Mary speaks to the innkeeper, who confirms that her brother was there, but she doesn't tell John this. They take two rooms at the inn and during the night a man tries to attack Mary, before being frightened off by her screams. She elects to spend the night with John.
The following morning, John realises that Mary has not been telling him the truth when the innkeeper repeats that he did see her brother and gave him directions to 'Norman Arches', a local ruin shown in the photos. While walking there, they find Ernest's handkerchief, which she tries to hide at first. She finally admits that she has been protecting her brother, afraid of his possible involvement in the theft of a precious diamond. While exploring the ruins, John realises that he drove through there on his way back from his holiday. Shortly afterwards they find a dead body, which the police identify as that of a notorious thief.
The police put out an alert for Ernest on suspicion of murder and soon catch him. Ernest initially denies ever being near the ruins, but the evidence is so damning that he eventually confesses. He had given the combination of the safe to the thieves, but later had a change of heart. Going to their rendezvous point, he had placed the camera to capture evidence of the thieves, but it photographed their fatal struggle instead. When his position was discovered by the murderer, Ernest fled, dropping the camera into John's car as he drove past. The Coroner doesn't believe Ernest and keeps him in custody.
Now in love with Mary, John is anxious to find a way to get Ernest released. He is visited by the policeman who first interviewed Mary about her missing brother. The two go into John's darkroom and the policeman quickly deduces that the man who stole the camera must have got stuck in the window frame by the broken sash and that the camera must have been left behind. John finds the camera, but becomes suspicious of the man's speedy deductions, especially as the sash has now been fixed and so he couldn't have known that it was broken on the night of the theft. John realises that the man is not a policeman but the murderer. The two struggle, but eventually the criminal is apprehended. Ernest is released and takes a photograph of John and Mary, who are now engaged to be married.
