The novel's plot is relatively simple. A French adventurer, tired of his soldier-of-fortune exploits in India, decides to comfortably settle down by marrying a stupendously rich Indian princess (the Begum of the title), but come a few decades, and their lineage ends without an issue, so the Begum's enormous fortune (the titular half billion francs) passes to the adventurer's two distant relatives, and they cannot be more unlike each other. One is Dr. Sarrasin, a gentle French physician, dismayed at the state of public health of the time, and the other is his distant cousin, Prof. Schultze, an already wealthy German industrialist who has built his fortune on the arms production. Both men decide to use their share of inheritance to build an utopian city as each of them imagined it.Both of them managing to lease large plots of land from the US state of Oregon (on the exterritorial terms not unlike of the Hong Kong lease)note , Dr. Sarrasin proceeded to build France-Ville, a modern, exemplary city with the emphasis on public health and healthy and sanitary living standards, while Prof. Schultze's Stahlstadt was an industrial powerhouse, built on the lines of militaristic discipline and split-second Prussian precision. Separated by the Cascades, both cities managed to happily ignore each other for several years. Not all is well, though, and the citizens of France-Ville become progressively suspicious of their militaristic neighbors, especially given Schultze's favorite racist rhetorics about superiority of the "Saxon" race over the "Latin" one, to which he was given even before his inheritance.Cue Marcel Bruckmann, the novel's true protagonist and the childhood friend of Dr. Sarrasin's son Octave. The brave and patriotic Alsatian, still holding the grudge over his homeland annexation during the Franco-Prussian war, infiltrates the fortified and compartmentalized Stahlstadt (passing himself as a Swiss) and manages to rise in its para-military hierarchy high enough to be admitted into the mysterious Tower of the Bull, the center of Schultze's power. Just as he becomes an assistant to the fortress city's dictator himself, he learns of the terrible secret — yes, all those years Schultze did plan and prepared to destroy the Ville-France, and he planned to use Weapons Of Mass Destruction to do this — his plans were first to bombard the neighboring city with incendiary shells, and then freeze and suffocate what remained with gas shells filled with liquid carbon dioxide.Marcel manages to warn his friends about this, but is discovered and has to hastily escape the city, and his warning was late anyway. Just as he returns to France-Ville, Schultze finishes his super-cannon and attempts to shell the city. Luckily, he was wrong in his calculation, and the cannon, instead of sending the shell some 40 miles into France-Ville, accidentally puts it into orbit. The heroes' city is saved, but knowing of the fate envisioned for them, its citizens begin to hastily militarize their previously peaceful city, before Schultze has the chance to repair his cannon and attack again, only to find that the attack doesn't come. Marcel and Octave, venturing to check what's going on, find an empty and desolate Stahlstadt, virtually left by its populace — an accidental explosion of Schultze's gas charge left him dead and frozen in his secret office, and because he ran his city as an absolute dictator, designating neither a heir, nor the deputy, it simply ground to a standstill without his orders. The heroes then return to their home city, deciding to utilize the Stalstadt industry to protect it from any future threats.Aside from being basically an Author Tract and providing several Unbuilt Tropes, the novel is notable in that it is one of the very few Verne's collaborations published during his lifetime. note It comes from a manuscript by the Paris Commune official and exiled revolutionary Paschal Grousset, who was living in the US at the time and was in bad need of an income. Grousset sold his manuscript to Verne's publisher Hetzel, who felt that it would sell better under a different name and that it needed a serious editing anyway, so he passed it to Verne for a rework. Verne rewrote the draft as he saw fit, and Hetzel published it under his name. In fact, this arrangement was utilized several times more, until Grousset was pardoned and could return to France during the Third Republic. He then built his own literary career as a famous adventure writer Andre Laurie.
The novel provides the examples of:
- All Germans Are Nazis: An Unbuilt Trope — Shultze is a proto-Nazi in every imaginable way, but it was a good half a century before the actual ones appeared.
- Author Tract: Both Verne and Grousset were French nationalists and used the novel to jab Germany after the lost war.
- "Blind Idiot" Translation: The novel's oldest and most well-known English translation is one of those that gave Verne a bad name in the English-speaking world.
- But Not Too Foreign: Marcel Bruckmann, the novel's protagonist, is an Alsatian, that is, a French German so to speak. He also passes himself as a Swiss while in Stahlstadt.
- Dystopia: Stahlstadt is clearly a Take That at the Prussian militaristic tradition and the German arms industry of the pre-World War I era (to the extent of giving off Putting on the Reich vibes, despite being written many decades before this trope came in full force).
- Enfant Terrible: Octave, who's a weak and indecisive playboy, and needs a strong guiding influence.
- Emperor Scientist: Prof. Shultze, who's a chemical engineer by his main trade.
- Fantasy Conflict Counterpart: The two competing cities are expies for France and Germany. Verne made no bones on where his sympathies were.
- Germanic Efficiency: Stahlstadt is operated on split-second precision and unthinking following of orders. If you doesn't like it — well, the train's waiting. If, however, you agree to play the game, competence is rewarded with rapid promotion.
- Germanic Depressives: Though comfortable and efficient, Stahlstadt is still quite a gloomy place.
- Guile Hero: Marcel.
- Load-Bearing Boss: Shultze runs his city personally and never employs any deputy who knows all of his secrets, so when he's killed in the accident, nobody knows what to do and the city grinds to a standstill.
- A Nazi by Any Other Name: Shultze. Long before any actual one, though the tradition that eventually bore the NSDAP certainly already existed.
- Nice Job Breaking It, Hero: While working as a mine engineer in Stahlstadt, Marcel reports to his superiors of the black damp — a deadly accumulation of several suffocating gases, mainly carbon dioxide — that killed several miners. Schultze's apparently had read this report, and it probably gave him an idea of his gas shells.
- Politically Incorrect Villain: Schultze's quite unabashedly racist, and publishes tracts about the "Latin" race's degenerate character.
- Real Life Writes the Plot: The novel was inspired by a disastrous French defeat in a Franco-Prussian war.
- Stock Yuck: Bruckmann was said by Verne to hate the stereotypically German fare of sausages with sauerkraut and beer. Ironically, these are staples of the Alsatian cuisine too, and, in the form of choucrute garnie, managed to become an all-France favorite nowadays.
- Unbuilt Trope:
- Weapons Of Mass Destruction: Shultze's gas shells, for which Marcel himself might've unwittingly give him an idea, after he reports a blackdamp (a carbon dioxide accumulation) in one of the mines while he worked there. Also, an Older Than Radio example of Recursive Ammo: a shell packed with small cannons firing incendiary bombs, designed to burn a city to ash with one shot.