The Secret Partner

John Brent is introduced as an executive working for Charles Standish at a shipping company's London head office. His marriage is experiencing problems due to financial troubles, causing his wife Nicole to leave him. Nicole suspects Brent is unfaithful and involved with another woman, Clive Lang.

It is revealed that Brent's real name is John Wilson, and he was previously imprisoned for embezzlement. After his release, he chose a dentist, Ralph Beldon, who also worked at the prison, which raises suspicion about Brent's background.

Beldon is being blackmailed by a masked stranger who demands he help with Brent's blackmail. The stranger plans to send Beldon a package of sodium pentothal to drug Brent during his dental appointment.

During Brent’s dental appointment, Beldon injects him with the truth serum and demands the combination to Standish’s safe. While incapacitated, Beldon makes impressions of Brent's keys to facilitate a theft of the safe’s contents.

Using the made impressions, the thieves steal £100,000 from Standish's safe, which contains more money than usual due to the shipping schedule. The robbery occurs without signs of forced entry.

Detectives Hanbury and Henderson investigate the theft. They find no signs of forced entry, and only Brent and Standish have the keys and knowledge of the combination. Traces of clay on the keys indicate they were copied.

Henderson suspects Brent of being involved and possibly aiding the thief, while Hanbury remains cautious and insists on a thorough investigation. Brent is brought back to England for questioning, and Standish is also questioned due to motives related to a promotion.

Brent notices the police finding evidence of copied keys in his apartment. He manages to distract the detectives and escapes, beginning to investigate on his own and attempt to mislead the police.

Realizing he was likely drugged by Beldon and was involved in a complex scheme, Brent confronts Beldon at his home, holding him at gunpoint until Beldon confesses to orchestrating the blackmail and theft.

Brent demonstrates to Nicole his disguise used on Beldon, revealing that he secretly planned the entire scheme to frame himself, eliminate Beldon, and secure the stolen money for himself and Nicole. However, she no longer trusts him.

Nicole, feeling betrayed by Brent's actions and the fake breakup, falls in love with another man and decides to leave him permanently, rejecting the stolen money and his manipulations.

Brent returns the stolen money anonymously. Hanbury, recognizing Brent's cleverness and lack of malicious intent, chooses not to prosecute him. The police chief wishes Hanbury a happy retirement as Brent walks away alone, reflecting on his complicated life.