West and Gordon are in San Francisco investigating a potential threat to national security which involves a vanishing seaside bar called the Mermaid Tavern, a dragon-like figure attacking ships, the imminent arrival of Admiral Farragut on a ship full of explosives, and the gem studded compact of beautiful, mysterious Dominique (Jocelyn Lane). Everything leads back to the Marquis de La Mer (John van Dreelen), a mad genius who plans to turn all the world's oceans into an underwater kingdom under his control.
Tropes present in this episode:
- Blow Gun: How the woman in the mermaid costume knocks out Jim during the Cold Open.
- Cassandra Truth: Unfortunately for the agents, Lt. Keighley finds it impossible to believe their story about dragons, mermaids, vanishing taverns, etc., and absolutely refuses to lend them any aid.
- Changing Clothes Is a Free Action: Artie changes his costume completely, dons a wig and applies a false mustache in the space of about 3 seconds.
- Despotism Justifies the Means: The "Marquis de La Mer" wants to establish the first entirely water-based kingdom and intends to blackmail the world into giving him the Pacific Ocean by threatening to destroy every ship which sails on it.
- Force-Field Door: Yet another invention of the Marquis'. It's meant to keep unwanted visitors out (or in) by instantly disintegrating anything which touches it. Jim and Artie, of course, soon figure out how the activation mechanisms work and use the knowledge to their advantage. No prizes for guessing what happens to the Marquis in the final act.
- Historical-Domain Character: David Farragut, the U. S. Navy's first admiral, happens to be aboard the next ship targeted for destruction.
- Homing Projectile: The "dragon" is actually a floating bomb - i. e. a torpedo - which can't be guided remotely but is instead designed to home in on the special compact Dominique carries.
- Large Ham: John van Dreelen plays the Marquis with lipsmacking relish, especially when he walks his men through the preparation of the torpedo for Jim's (and the audience's) benefit. "CONNECT THE WIRES TO THE DETONATOR!"