 Hanna Crandall, a witness in a trial against an intelligence operative of a Communist nation, panics. She hurriedly disguises herself and buys a train ticket from New York to Quebec. She s not aware that assassins in the employ of that country are on the train with her. Hanna also befriends a male passenger, who realizes something is wrong but has trouble reaching out to Hanna. Erskine and Rhodes figure out she s on the train and manage to get aboard. Now the FBI agents must protect her before the killers can complete their mission.