Two penniless English detectives Flynn and McPherson, portrayed by Hans Albers and Heinz Rühmann, travel to the Brussels World’s Fair, hoping to find work. They arrive with no train fare, boarding the night train to Brussels on open tracks, and are astonished when the conductors—and two shady passengers who bolt from their compartment—mistakenly treat them as the legendary duo. Flynn and McPherson seize the now-empty compartment “for investigative purposes” and set about questioning the nearby travelers, including two seamstress sisters, Mary Berry and Jane Berry, as they head to claim the inheritance of their wealthy, recently deceased uncle.
The pair check into the prestigious Hotel Palace in Brussels, and in the luggage left behind by the fleeing passengers they uncover hidden compartments containing large sums of money in three currencies and a set of encrypted plans. A criminal duo, Madame Ganymare and Lapin, also stay at the hotel, and they break into the detectives’ room, discovering only a receipt from a costume shop. They threaten to expose Flynn and McPherson as impostors unless the receipt is swapped for the valuables.
News of “Sherlock Holmes” and his companion’s arrival spreads, and the police summon them—not to arrest, but to help solve a perplexing case. Four rare Mauritius stamps have vanished from the World’s Fair and been replaced with forgeries, and a ransom note demanding 600,000 francs has already surfaced. The uncle’s estate reveals that 200,000 francs in cash are missing. Holmes deduces that Berry orchestrated the theft, paid the criminals with the missing cash, and even operated a secret counterfeiting workshop in the castle, where he forged stamps. When Berry tried to back out, the gang sent a threatening letter, and he died of heart failure.
Armed with this insight, the detectives trace the gang’s hideout to a Brussels pawnshop, using the threatening letter as their map. Back in Brussels, they discover they are now wanted by the police. [Holmes] uses a wanted poster to infiltrate the gang, feigning an eagerness to join them. Lapin, the gang’s leader, turns out to be cunning and dangerous. Flynn and McPherson are captured, and Lapin fiddles with his pocket watch in a way that raises Holmes’s suspicions. A staged brawl erupts, the watch is snatched, and the two are locked in a coal cellar. They barricade themselves inside, and when Lapin finally discovers the theft, he orders the door rammed open. At the last moment, Mary Berry’s warning hands the police the opening they need, and the entire gang is arrested.
The trial that follows sees Flynn and McPherson accused of impersonation and fraud, accused of deceiving both the police and the World’s Fair director. They defend themselves by arguing they never explicitly claimed to be Holmes and Watson, and they insist their actions served justice: they exposed the counterfeiting ring, uncovered three international bank robberies, returned the stolen money, handed over the decoded plans, and recovered the stamps—found hidden in a secret compartment of Lapin’s pocket watch. A stamp expert confirms the stamps are genuine, and the charges of impersonation begin to look shaky.
In a final, tongue-in-cheek twist, Arthur Conan Doyle reveals himself in the courtroom. Amused by the entire affair, he declares that Holmes and Watson are fictional, retroactively granting Flynn and McPherson permission to use their names—on the condition that he may write a book about them titled The Man Who Was Sherlock Holmes. The trial is dismissed amid thunderous applause. Flynn kisses Mary Berry goodbye, while McPherson shares a kiss with Jane Berry.
