The trendy elegance of director Douglas Sirk's later big-budget soap operas is nowhere to be found in Sirk's Atomic-Age melodrama Mystery Submarine. MacDonald Carey stars as Brett Young, a U.S. undercover agent whose job it is to prevent atomic secrets from falling into the wrong hands. Marta Toren co-stars as Madeline Brenner, a woman of mystery who is seemingly in cahoots with enemy agents. The film's "maguffin" consists of the top-secret info stored within the brain cells of scientist Adolph Guernitz (Ludwig Donath). The titular mystery submarine figures into the film's climax, which takes place just off the coast of Mexico.