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.