The Admiralty receives a letter from a crank inventor offering the patent of his submarine for £10. Lt Pimple, sent to investigate, is shown round the vessel by its inventor and agrees to buy it for 25 shillings. A foreign spy in disguise offers to 'mind' the vessel while Pimple takes the inventor to the Admiralty, and in his absence steals the submarine with the aid of two co-spies. On returning to find the vessel gone, Pimple dons a diving suit to investigate. After a fight on the sea bed with one of the spies, Pimple is eventually trapped inside the submarine and left to his fate by the other two villains. He sends a note to the surface inside a fish, which is caught by a fisherman and handed to the captain of HMS Invincible [a wooden barge], who returns a message by the same medium. Pimple emerges on the surface wearing his diving suit, having declined to await rescue. The Invincible gives chase to and fires at the spies in their rowing boat. The captured spies are hauled on board.

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