From the moment Nancy Drew boards an ocean liner leaving for New York, she becomes involved in a new and dangerous mystery. A man on the pier gestures to someone on board in sign language.

BEWARE OF NANCY DREW AND NE, he signals.

Who is NE? Can it be Nelda Detweiler, a young woman who shares a cabin with Nancy, Bess, and George? Mystery and intrigue follow the girls across the ocean; but with her usual ingenuity Nancy solves the mystery and defeats her enemies before the ship reaches New York.