In 1947, Inspector Garvey Denis Greene of Scotland Yard suspects Michael Lanyard Gerald Mohr, the reformed jewel thief known as the Lone Wolf, of orchestrating the theft of the priceless diamonds called the Eyes of the Nile. Lanyard denies involvement, claiming he is in London with his butler Claudius Jamison Eric Blore to write a book on the jewels and that he was in New York when they were stolen.
Lanyard and Jamison are short of funds and when Ann Kelmscott Nancy Saunders, the daughter of wealthy gem collector Sir John Kelmscott, invites them to the family estate, they agree.
Sir John confides that he is in desperate need of money and asks Lanyard to arrange a confidential loan with part of his jewel collection as collateral.
Jamison tells his master that Lily, the maid Queenie Leonard told him the butler, Henry Robards, is heartbroken because Iris Chatham Evelyn Ankers has run off with Monty Beresford Alan Napier who financed the lavish stage production that launched Iris to stardom.
Back at his hotel, Lanyard receives a call from Iris, inviting him to the theater that night where she asks him to stay away from the Kelmscotts.
At her apartment, Robards begs her for a reconciliation, but she demands something first.
Lanyard decides to accept Kelmscott's offer and arranges to meet Bruce Tang, a gem dealer the next morning.
Kelmscott gives Robards a packet of jewels to deliver to Lanyard but he steals the Eyes of the Nile for Iris.
David Woolerton Richard Fraser, Ann's fiancé, asks him for a ride because he wants to spy on Lanyard.
At Tang's shop, Inspector Garvey has followed Lanyard.
When Robards' car rolls to a stop, the butler is dead behind the wheel, the jewels still in his possession but the Eyes of the Nile are gone.
Woolerton claims Robards pushed him out of the car on the outskirts, and said he suspects Lanyard was the murderer.
Lanyard thinks Kelmscott was being blackmailed by the thief who sold him the diamonds, and flees the police and proceeds to Iris' apartment.
She has just persuaded Monty to get the diamonds from Lanyard.
At the theater, Lanyard discovers the diamonds are sewn into her mink coat but before he can leave her dressing room, Iris enters and admits that Robards had given her the diamonds.
While Lanyard phones Jamison, Iris sneaks out an open window and heads to the airport to catch a flight out of the country.
At the airport, Lanyard confronts Iris and turns her over to the waiting Garvey but Monty arrives, wielding a gun and declaring Iris planned the entire robbery and then killed Robards for the diamonds.
When Jamison brings Lily and Iris' mink, Lanyard extracts the Eyes of the Nile from the coat's linings, proving that Iris was the thief and murderer.
