Short summary - Facing the Flag or For the Flag
Jules Gabriel Verne
Brilliant scientist Tom Rock is the inventor of the fulgurator, a weapon of the greatest power. Embittered by his failures, Rock sells the fulgurator to the leader of a group of scoundrels and crooks from all over the world... and becomes his prisoner himself.
Inventor Tom Rock creates a destructive weapon that consists of two elements: an explosive and a device into which the substance is poured (prototype of a rocket). The eccentric and pathologically distrustful Tom Rock tries to sell the invented weapon to different states, demanding astronomical sums for it, but the governments of all countries refuse him.
The last time he turns to the United States of America, but the US government, considering the inventor mentally deranged, places him in the Healtful House mental hospital, hoping to bargain with him when he recovers. Soon from there, Roca is kidnapped by an unknown Count d'Artigas and taken away on his schooner. Together with Rock, his caretaker Gaydon, whose real name is Simon Hart (French engineer), is also kidnapped.
Pirates bring them to the remote island of Back Cap, which is part of the Bermuda group. Count d'Artigas turns out to be Ker Carrage, the leader of the pirates who want to lure his invention from Tom Rock in order to become invulnerable to any armed forces with his help. They bribe him with their treasures, and Rock makes his invention.
Simon Hart manages to inform the world about the impending danger. The English military submarine "Sword" under the command of Lieutenant Devon sails unnoticed to the island. They pick up Simon Hart, capture Tom Rock and try to sneak away unnoticed, but the pirate's sub notices them and sinks them, ramming them with their heavy hull. Only Rock and Hart manage to escape.
Several warships approach Back Cap to bombard it and land troops, but Tom Rock sets his weapons into action and destroys the English cruiser. Then a French ship comes forward, raises the flag, and the military band on the ship begins to play the French anthem. Thomas Roque, a Frenchman by origin, having seen the flag of his homeland, feels guilty before the world and his homeland. He blows up the island with all the pirates. When the soldiers descend on the remains of the island, they find the only survivor - Simon Hart.