The Paris-Rome express departs with a full complement of passengers. American film star Asta Marvelle is accompanied by her indefatigable if over-zealous publicist Sam. Millionaire philanthropist Alastair McBain is travelling with his secretary Mills. M. Jolif, the head of the Sûreté, or French police, is travelling alone. He shares a compartment with Mr Poole, a nervous man who never loses sight of his slim attaché case. Equally nervous are George Grant and the married Mrs Maxted, fearful that her husband might discover their affair. All these are watched intently by Mr Zurta and his colleague Tony, who board the train at the last minute.

Poole has a casual conversation with M. Jolif, who tells him that he never forgets a face. The guard reveals his identity to a nervous Poole shortly afterwards. Asta tells Sam that she'd rather not be seen as a celebrity for the duration of the train. George is horrified to spot a neighbour, Tom Bishop, and pretends that he's sharing his compartment with Poole. The latter, who is trying to change his compartment anyway, is happy to go along with the charade.

Mills reads a newspaper article about a missing Van Dyck painting, which McBain had tried to buy. He is still angry at being turned down.

Once Bishop and Poole leave, George quickly thrashes out a story with Mrs Maxted, and they agree to separate for the duration of the trip.

Tony and Zurta compare their findings, and it becomes clear that they're looking for the stolen painting. Tony later bumps into Asta, who turns out to be an old flame. They reminisce nostalgically about their life four years earlier, before she became a star.

Mrs Maxted moves into another compartment with a middle-aged spinster, who is nervous about the possibility of a train crash. Meanwhile, George tries to persuade Poole to leave his compartment, reluctantly agreeing to help him find another one.

Bishop invites George and Poole to a game of poker in the dining car, roping in Zurta and Tony. Poole recognises Zurta and looks nervous. He puts his case on the floor by the seat, where it is inadvertently swapped with an identical case of papers. Mills takes the latter back to McBain's compartment. McBain opens it and discovers the stolen painting.

Poole wins the poker game handsomely, and retires to his compartment. He begs George not to evict him, and when the latter insists, he knocks him out with a bottle. Attempting to leave, he bumps into Zurta and Tony. Zurta interrogates him about the painting, revealing that they're criminal associates. Poole swears that he would have handed over a share of the proceeds. Zurta kills him and calls on Tony to help him make it look as though he's fallen asleep.

Tony goes to Asta's compartment and says he's in a difficult situation, but she says she'll stand by him. Mrs Maxted, believing Poole to have left as planned, goes to George's compartment and finds him blearily recovering and Poole's body. A guard offers her assistance, sees the body, and summons the chef de train.

M. Jolif begins investigating Poole's murder. He accuses George of being involved, since the two were seen together earlier. George explains the situation with Mrs Maxted.

Mills discovers that his case was switched, and McBain berates him for his carelessness. Zurta overhears this, and sneaks into McBain's compartment in search of the painting. He is caught by McBain and Mills, and threatens them with a gun. Mills successfully traps his hand in the door, disarming him - but the gun goes off, shattering a mirror and revealing the Van Dyck behind it. Mills accuses McBain of taking the picture for himself. He attempts blackmail, but carelessly hands over the picture when McBain requests it.

McBain gives the picture to M. Jolif, saying that he found it hidden in his compartment. Zurta attempts to accuse McBain of stealing it, then falls out of the train to his death.

The train arrives in Rome. George tells Bishop that Mrs Maxted is returning home at once, and asks for his discretion. While Sam organises a welcome reception for Asta, she is sneaking off with Tony via a different exit.