At Nice Carnival, two costumed men have a drunken but pleasant social encounter - but when they meet again, insurance agent Harwood is startled to find that Heinrich Wagner is dead - and even more surprised when the body vanishes after he fetches help in the form of his newly-assigned partner Caryl Fenton. She is more concerned that they catch their train.
On the train, Harwood's disturbing dream climaxes in a genuine train crash. Climbing through the wreckage, he discovers Wagner's body. Discussing the crash with French detective Monsieur Turbé, they agree that it was probably engineered deliberately to make Wagner's death look accidental.
In Paris, Harwood and Caryl visit a flat at an address on a scrap of paper owned by Wagner. Harwood is suspicious when the man who lives there claims no knowledge of the crash, despite clearly visible newspaper headlines.
That night, Harwood and Caryl break into the flat, only to discover that it has been vacated. They find an invitation to "Axel Hoyt and party" inviting them to a London Guildhall banquet, dated five years earlier. They are shot at from the building opposite, but successfully escape.
In London, they surreptitiously obtain a 1931 Guildhall banquet photo. They recognise Wagner and Hoyt, who are sitting with a haughty-looking couple, and a man seen from the rear, holding a cigar. The woman is wearing a designer gown, and Caryl traces it to one Miss Elizabeth Wentworth of Buckley.
They sign up for the Buckley Whist Drive, where they have to move tables in a specific direction depending on each game's outcome. Harwood eventually ends up sharing with Wentworth. He claims to have met her at the Guildhall banquet in 1931, and that Axel Hoyt is a mutual friend. She says he died three years ago.
At the Buckley registrar Harwood and Caryl obtain Hoyt's death certificate by pretending that they want to name their son after him but want to check the spelling. Spotting the signature of a Dr Evans, Harwood pretends he's an old friend with whom he's lost touch. The registrar gives them the address, explaining that he's retired thanks to an unexpected financial windfall three years ago.
But Dr Evans he left in a hurry fifteen minutes earlier to catch the train. Harwood and Caryl give chase in their car, only to see the train crash into a lorry parked across the tracks.
Harwood meets Turbé, who hands him Wagner's cufflink, and its matching pair, found on Dr Evans' body a few minutes earlier. Harwood gets a phone call from Buenos Aires, revealing Hoyt as a gun-runner. Caryl notices a leaflet for a Pilgrims of Peace meeting, sporting a logo matching the cufflink
At the meeting, Wentworth spots Harwood in the crowd, and points him out to Hoyt. A fight breaks out, but Harwood and Caryl manage to escape thanks to an exhibit containing a live grenade.
They follow their quarry to a train, meeting Turbé on the way. Harwood and Caryl go to confront them in the dining car, with Turbé occupying a nearby table. Harwood identifies Hoyt as an Argentinian criminal who faked his own death rather than face extradition. Hoyt admits this, but denies the murders.
Harwood notices something oddly familiar about the way Turbé is holding his cigar, and realises that he's the other man in the Guildhall photo - and the only one with the motive and opportunity to commit the murders. Turbé quietly moves to the next carriage, detaching the dining car on the way.
Realising they have a common enemy, Hoyt tells Harwood everything. The detached carriage enters a tunnel, into the path of an oncoming train. Harwood and Caryl jump to safety, the others are less fortunate.
Back in London, Turbé watches a newsreel about the crash, which names the dead as including Harwood - who is in fact sitting behind him. Fleeing, Turbé is shot, collapsing in front of the screen as the newsreel asks when the wrecker will be brought to justice.
Harwood and Caryl return to the Buckley registrar to get married.
