A patient, Ellen Summers, watches as a van pulls up to a private sanatorium to make a delivery. She ducks into the back when the driver isn’t looking. Later, Ellen gets aboard a train and enters the cabin where a bandaged Peter is going over his papers. She’s been looking for him and asks for his help, and Peter reluctantly hears her out. Ellen explains that someone has followed her on the train and will lock her up again. After drawing the blinds shut, Ellen says that she’s a photographer and took a photo of George Wilson.
A woman comes in suddenly. Ellen ducks down and the woman apologizes and goes on her way. Once she leaves, Ellen explains that the woman is Miss Beck, Dr. Trevors’ assistant, and Dr. Trevors has kept Ellen locked up at the sanatorium for months, driving her mad with injections. She tells Peter that she went to the police but they didn’t believe her, so she went to Peter’s house and saw him drive to the train station. Ellen informs Peter that George Wilson, an innocent man, will be executed the next day. Wilson was a young farmer and Ellen his girlfriend, and Wilson worked at the farm attached to the sanatorium estate. According to the newspapers, Wilson lost his temper and shot a police inspector, Hunter. Ellen has the photo that proves Wilson didn’t shoot Hunter.
The train arrives at the station and Diane is there to meet Peter. He goes on ahead and starts to warn her about Ellen, only to discover that she has apparently slipped away in the crowd. As they drive home, Peter checks Diane’s newspaper and sees an article about Wilson. He wonders if Ellen was right and Wilson is innocent, and has Diane drive him to the prison. Peter slips in invisibly and tells Wilson that he’s there to help. Wilson confirms that he was at the races the day of the murder but couldn’t prove it. He confirms that Ellen was his girl and was at the races with him, but walked out on him when he needed her testimony. Peter promises to do what he can for him if he’s innocent. The guard comes back in with the governor, who informs Wilson that his appeal has been refused. Peter whispers to Wilson that he’ll be back and leaves with the governor.
Next, Peter makes an appointment with Dr. Trevors and Diane drives her brother to the sanatorium. Trevors is eager to hear about Ellen’s whereabouts, explaining that she suffers from amnesia and prosecution complexes and fled. When Peter explains his interest, Trevors tells him that she’s deluded and that Wilson tried to make use of her to create an alibi for himself. The doctor admits he was the reluctant witness against Wilson in the murder trial and Peter leaves, apparently satisfied. As he leaves, however, Beck comes in. Peter continues out on but Beck warns Trevors that Peter must have recognized her from the train.
Peter has Diane pull over outside the sanatorium and tells her about Beck. He goes back invisibly. Meanwhile, Trevors and Beck go to the room where they have Ellen tied up. They gag her and prepare to give her a sedative to keep her from escaping. However, Peter bursts in, punches, Trevors, and frees Ellen. Trevors insists that Ellen was insane and attacks Peter with a chair, but Peter yanks it away from him and hits the doctor with it, and then leaves with Ellen.
As Diane drives them away, Peter asks about the proof that Ellen said she had. She explains that she was at the races with Wilson and took a photograph of her boyfriend. Ellen hid it at her apartment and didn’t have time to give it to the police. When the police arrested Wilson, Trevors called her and he offered to drive her to the police station. Ellen got nervous at the last minute and hid the negative behind a tear on the wallpaper. She showed him the photo and Trevors asked to see the negative, and he held her while his assistant searched the apartment but failed to find the negative. Then they gave her an injection and took her to the sanatorium.
Diane drives to Ellen’s old apartment and Peter and Ellen go in while Diane calls Sir Charles. The woman, an artist named Mrs. Willis, refuses to let Ellen in and Peter invisibly shoves the door open. Willis has moved everything around and bricked over the fireplace. While Willis calls the police outside, Peter uncovers the bricked-over fireplace and Ellen directs him to the tear in the wallpaper. Ellen goes to Willis and confirms that she papered over the old wallpaper. Willis watches in surprise as Peter picks up a trowel and uses it to slice open the wallpaper and pull out the negative. Ellen apologizes for frightening Willis and explains that Peter is there, and she tells them to get out. When Ellen tries to leave, Willis tries to hold her for the police. Peter sends her on, apologizes, and puts the paintings back.
Trevors and Beck are waiting for Ellen below and knock her out with a sedative. When Peter comes down, there’s no sign of Ellen and Diane is just coming back. Meanwhile, Beck searches Ellen’s purse, unaware that Ellen has slipped the negative into her shoe. Trevors pulls over at a stoplight and Ellen leaps out. She’s hit by a passing car and knocked out. As a crowd gathers, Trevors tells the officer on the scene that Ellen is a mental patient and orders him to help put her in his car. The officer refuses and tells them to wait for the ambulance.
Wilson waits nervously in his cell.
Diane and Peter check with the police but there’s no word of Ellen, Peter decides to go see Sir Charles. Peter tells him about the negative but admits that Ellen has it and Trevors and Beck have disappeared. Sir Charles warns that they’ll need to see the evidence and Peter admits that he couldn’t recognize Wilson from the negative. With less than six hours left, Sir Charles agrees to call the authorities and intensify the search.
At the hospital, Trevors demands to see Ellen but the nurse refuses to let him without the permission of the house surgeon.
With dawn and Wilson’s execution coming, Sir Charles finally gets a call informing him that Ellen is at the hospital.
When the constable watching Ellen is called to the telephone, Trevors sneaks into Ellen’s room and tries to get through to the injured woman. She claims that she can’t remember what happened to the negative, and the nurse comes and demands to know why Trevors is disobeying procedure. When he refuses to leave, the nurse goes to get someone. Trevors continues pressuring Ellen and she finally says the negative is in his shoe. He gets it just as the constable comes in and escorts him out.
Peter and Sir Charles arrive and Peter goes on ahead. He finds Trevors in the hallway, preparing to burn the negative, and stops him just in time. When the constable arrives, Peter turns Trevors over to him and takes the negative to Sir Charles, and tells him to call the home secretary and stop the execution.
Wilson is being escorted out of his cell when Peter arrives and tells the prison governor to wait until the home secretary calls. The man agrees to call the commissioner, who says that there’s been no reprieve. A desperate Peter locks Wilson and the guards into a cell, and the home secretary calls moments later to tell the prison governor that Wilson has received a reprieve. Trevors has confessed that he was using the sanatorium for his racket, and the police inspector found out so the doctor murdered him. A grateful Wilson thanks Peter for his help.