Dr. Clive Riordan, Robert Newton, a wealthy London psychiatrist, learns that his wife Storm Riordan, Sally Gray, is having an affair with Bill Kronin, Phil Brown, an American. This discovery sparks a cold, calculated plan to execute the perfect murder of Kronin, driven by jealousy and a desire for control over a situation spiraling out of his hands.
Riordan kidnaps Kronin at gunpoint and confines him for months in a hidden room tucked behind a garage in a bomb-damaged sector of the city. With no solid leads, the authorities search in vain, and the case slowly fades from the public eye. Riordan convinces himself that keeping Kronin alive for so long will erase any connection to him once Kronin disappears, allowing the crime to stay unsolved and his motive to remain buried beneath the surface.
During one routine visit to bring Kronin food, Riordan is unknowingly trailed by his dog Monty, a detail that tightens the stakes. In a chilling moment of brutal resolve, Riordan reveals his plan to Kronin: to kill him and dissolve the body in an acid bath. He grows increasingly paranoid about Monty giving away the secret, and for a time contemplates using the bath prepared for Kronin on the loyal dog as well. Yet Kronin refuses to yield and manages to protect Monty from harm.
The plot thickens as Riordan’s meticulously laid trap begins to crack. He has even prepared hot water bottles filled with acid to ensure a thorough disposal. The tension peaks when Superintendent Finsbury, from Scotland Yard, visits Riordan’s office. Naunton Wayne plays the perceptive investigator who suspects something is amiss, especially after picking up little tells like Riordan’s American-sounding phrases and the way he interacts with an American—clues that keep gnawing at him.
Riordan’s plan nearly unravels when he overhears a conversation between Storm and Finsbury, prompting him to act with a desperate move: he poisons Kronin. But Kronin has learned enough from Monty to outmaneuver him—Monty somehow manages to pull the plug chain and drain the acid bath, foiling Riordan’s lethal plan. The police soon close in, finding Riordan’s car in the garage and tracing the clues to a hidden room where Kronin’s fate could have been sealed.
Confronted with the failure of his scheme, Riordan accepts the grim reality of a likely arrest, retreating to his club to await what comes next. In a dramatic turn, Finsbury arrives to reveal that Kronin survived, sparing Riordan from a murder charge and leaving the doctor to face the consequences of his actions in the end.
Storm’s visit to Kronin in hospital adds a subtle, uneasy note to the conclusion. She tells him she plans to travel abroad for a while, and as she departs, Monty suddenly leaps his lead and darts back to Kronin’s bedside to show affection. Storm offers a faint, knowing smile and steps out, leaving the two men to their uncertain, fragile future together.
