Intrigue and espionage, and the effects on the lives and futures of passengers aboard a Trans-Continental Express emerge, when a girl traveller (Margaret Lockwood) returning from a holiday, strikes up an acquaintance with a middle-aged English governess who, during the journey mysteriously disappears from her compartment.

The girl, seeking an explanation for the disappearance, is accused of hallucinating and is nearly convinced that her new friend does not exist.
However, further inquiries made among the passengers reveal the curious behaviour of a group of foreign government agents who are also travelling as passengers...