Nancy, Bess, and George make plans to travel to South America by boat, and her father buys her a new initialed brass-bound trunk from a local luggage firm.
Prior to departure, Nancy learns there are issues concerning the questionable quality of the merchandise.
The owner, a friend of Carson Drew, asks Nancy to investigate and to convince his daughter, Doris, to marry Henry Washburn, the son of his deceased business partner.
After the ship sails, Nancy discovers that an identical trunk was delivered to her cabin by mistake and her luggage is missing.
She vows to find the owner of the other trunk, return it and recover her own.
After locating the other girl, Nancy is puzzled as to why thieves keep trying to steal both monogrammed trunks.