The Unseen Enemy in this wartime meller is Nick (Leo Carrillo), the outwardly effusive manager of a San Francisco waterfront café. To make enough money to ensure his daughter Gen's (Irene Hervey) entree into society, Nick sells his services to a gang of foreign spies, who then use Nick's establishment as a rendezvous point. The plan is to covertly send out a Japanese vessel for the purpose of raiding and destroying American merchant ships. The spies' secret code is hidden in the lyrics of a song called "Lydia", which the unwitting Gen performs on request day after day.

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