On one of the cars of the North Express, traveling the route Montreal–Quebec, a young woman is murdered. By a lucky coincidence, detective Frank was among the passengers and took the case into his own hands. The murder weapon—a dagger with traces of blood—was discovered almost immediately; it lay nearby, close to the victim. The killer didn’t even bother to hide it. All that remained was to question every passenger on the ill-fated train, because each of them could have been a potential murderer.