John Brent is an executive working under Charles Standish at the London headquarters of a shipping company. His marriage faces turmoil because of ongoing financial struggles; his wife Nicole Brent, convinced that he is lavishly spending money on another woman, decides to leave him and begins a relationship with Clive Lang, a decorator they had hired.
What Brent's employer does not know is that his true name is John Wilson, and he previously served time in prison for embezzlement. After his release, he chose a dentist, Ralph Beldon, who happened to be the same dentist who performed dental work during his incarceration. Contrary to Brent's intentions, Beldon is blackmailing him, threatening to reveal his criminal past to the company unless he complies.
The plot thickens when a stranger, whose face is masked and voice altered with a dental insert, visits Beldon’s home. This stranger, aware of the blackmail, pressures Beldon to exploit Brent even further. Beldon receives a package containing sodium pentothal — a truth serum — and at Brent's next dental appointment, Beldon sedates him and demands the combination to the safe at his office. While Brent is incapacitated, Beldon plans to make copies of Brent’s keys, both for his home and office. The stranger promises Beldon a payment of £15,000 for his cooperation.
The day of the theft, the safe contains an unusually large sum of £100,000, owing to the scheduled shipments. The safe is forcibly opened without signs of forced entry; only Brent and Standish possess the keys and the combination. Brent has gone on vacation, and his keys are at the office. Evidence such as clay impressions on the keys suggests they were duplicated.
Detectives Superintendent Hanbury and Inspector Henderson take on the investigation. Henderson quickly suspects Brent, especially when no signs of break-in are found, and the only keys and known combination point to him. Meanwhile, Hanbury, who is nearing retirement and prefers a thorough investigation, doubts Brent’s guilt and insists on exploring all leads. They question various suspects, including Standish — who had motives related to a potential promotion — as well as Nicole, Lang, and others.
Eventually, the police locate evidence of keys being copied at Brent’s apartment. Brent, feeling the pressure, manages to divert attention and flees, attempting to uncover the truth himself. As he investigates, he begins to suspect he might have been drugged by Beldon, thanks to clues he pieces together. This suspicion leads him to Beldon’s residence, where, armed with a gun, he confronts and forces Beldon to confess his crimes. Brent then hands over the unloaded weapon to Hanbury.
As the story unfolds, it is revealed that Brent himself is the criminal mastermind. He had always planned to frame Beldon for the theft and manipulate the situation to his advantage. His aim was to eliminate the blackmailer and share the stolen money with his wife. The final twist reveals Brent’s cunning disguise and his deception of both the police and Nicole. He demonstrates his elaborate disguise to her, exposing that he had been the one orchestrating the whole scheme from the beginning.
Nicole, however, is no longer interested in the arrangement. She accuses Brent of exploiting her by faking their marital issues, then burdening her with Lang’s affection. She confesses she has fallen in love with another man and wants to leave him forever.
Feeling heartbroken and defeated, Brent decides to return the stolen money anonymously. As the story concludes, Hanbury calls Brent in to inform him that he suspects Brent’s involvement but chooses not to press charges, now that Hanbury is about to retire. Brent, having burned bridges and lost his personal life, walks away into solitude, a shadow of his former self, in a world where trust and morality have crumbled.
