The film centers almost entirely on the Rome Express, a train traveling between Paris and Rome. As it pulls away from a Paris station, Zurta, Conrad Veidt and his ally Tony, Hugh Williams, manage to board the carriage, both intent on reaching someone on board. Also aboard are McBain, Cedric Hardwicke, a wealthy businessman travelling with his brow-beaten secretary Mills, Eliot Makeham, and a painting by Van Dyck that McBain had previously tried to buy, now unrecovered and coveted enough to motivate anyone to take bold risks. The train’s eclectic mix is completed by an adulterous couple, an annoyingly sociable Englishman, a French police inspector, and Asta Marvelle, Esther Ralston, a fading American film star who travels with her manager/publicist, and a publicist named Sam, Finlay Currie.
Onboard tension centers on a single, prized object: the stolen Van Dyck painting. Poole, [Donald Calthrop], carries a briefcase he keeps close, and Mills watches the world warily from his seat—both men know the painting’s value and that Poole must be the one to deliver it if it’s ever going to leave the train. To complicate matters, Poole agrees to join a poker game, where he recognizes Zurta as one of the players, a realization that unsettles him. In a careless moment, Poole sets down his briefcase and slips, quite by accident, into Mills’s hands—the two have swapped cases without realizing the mix-up. The poker game ends, and the narrative begins to tilt toward a deadly clash of loyalties as the briefcases change hands.
Zurta, convinced the key to everything lies in Poole’s possession, follows Poole to his compartment, forces his way in, and confronts Poole. Poole offers to hand over the painting but discovers that he’s not in possession of the correct briefcase. A tense struggle follows, Zurta’s threats escalate, and Poole is killed as the train snakes onward, leaving confusion and fear in its wake.
Meanwhile, McBain discovers the truth in Mills’s world of cases: the stolen Van Dyck painting makes an appearance in Mills’s briefcase, the prize McBain has long sought. The arrival of the body triggers a police inquiry, and the inspector begins to interview everyone who crossed paths with Poole. Zurta learns that the briefcases have indeed been swapped and makes a frantic attempt to recover the correct case from McBain’s compartment, only to be blocked by McBain and Mills until the inspector arrives to take charge.
As the investigation tightens, Mills realizes the painting is hidden in McBain’s compartment and that McBain may have found it first. He tries to leverage the painting to blackmail McBain, but McBain outsmarts him and hurries to place the painting with the police, eliminating Mills’s leverage. The tension reaches its final crescendo as the suspicions of the train’s authorities fall on Zurta, who suddenly hurls himself from the moving carriage in a last-ditch escape attempt—and is killed in the fall. The Rome Express, its passengers left to weigh the cost of greed and secrets, continues its journey while the painting’s fate remains in the hands of the law.
