Plot
John Brent is an executive, working under Charles Standish at the London head office of a shipping company. His marriage is in trouble because he is always short of money; his wife Nicole, certain he is spending it on another woman, leaves him and takes up with Clive Lang, a decorator they have hired.
What Brent's employer does not know is that his real name is John Wilson and he was once imprisoned for embezzlement. When he chose a dentist after being released, it happened to be the same man who did dental work at the prison, Ralph Beldon. Brent is not cheating on Nicole, but Beldon is blackmailing him by threatening to reveal his true identity to the company.
One day a stranger, his face masked and his voice disguised by an insert in his mouth, arrives at Beldon's home. He knows about the blackmail and demands Beldon take further advantage of Brent. Beldon will be mailed a package of sodium pentothal. At Brent's next dental appointment, he is to inject Brent with this truth serum and demand the combination to Standish's safe at the office. While Brent is incapacitated, he must also make impressions of Brent's keys, both home and office. Beldon is promised £15,000 to be paid later.
The plan is carried out, and the safe is robbed on a day when it happens to contain more money than usual – £100,000 – because of the scheduling of the ships.
Detective Superintendent Hanbury and Inspector Henderson investigate the case. There is no sign of a break-in; Brent and Standish have the only keys and are the only ones entrusted with the combination. Brent has just left the country on vacation and his keys are at the office. Traces of clay on them indicate that impressions were made.
Henderson jumps to the conclusion that Brent is guilty and provided the impressions to the actual thief; Hanbury, who is about to retire and wants to leave things neat and tidy, is less certain and insists on a proper investigation. They question Standish, Nicole, Lang, and others, and have Brent brought back to England for questioning. Standish had a motive to hurt Brent: he had learned that Brent was likely to be promoted to replace him.
But at Brent's apartment, the detectives find evidence of keys being copied. Brent manages to distract them and flee. He then tries to investigate on his own, and also to mislead the police, until he finally realizes that he might have been drugged by Beldon.
He calls Hanbury to Beldon's place and goes there with a gun, threatening Beldon at gunpoint until the man confesses to all his crimes. Brent then hands the unloaded gun to Hanbury.
The viewer learns the truth when Brent returns home and demonstrates to Nicole the disguise he used on Beldon. Brent is the criminal and always intended to frame himself and then blame Beldon. In this way he would get rid of the blackmailer and he and Nicole would have £85,000 to share.
However, she is not interested. She feels he took advantage of her by faking the breakup of their marriage and saddling her with Lang's attentions, and now she has fallen in love with another man and wants to leave him for real.
Heartbroken, Brent returns the money anonymously. Hanbury calls him in: he has guessed Brent's tricks, including the "pentothal" that was actually some harmless liquid, but to Henderson's surprise, he does not now feel it would be worthwhile prosecuting Brent.
Henderson wishes Hanbury a happy retirement, and Brent walks away, now alone in the world.
