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After purchasing a riverboat from Scotsman Gillespie MacKenzie (Jack Livesey, Bret finds out that MacKenzie has already sold the same boat to seven other people--including his old nemesis, the combustible Modesty Blaine (Mona Freeman). Rather than fight over the matter, the eight owners decide to take the boat to Memphis for resale. En route, the members of the party are killed off one by one by a mysterious assailant...who seems to be deliberately saving Bret and Modesty for his final victims.