The novel focuses on the financial world of the Second French Empire as embodied in the Paris Bourse and exemplified by the fictional character of Aristide Saccard.

Searching for a way to reestablish himself, Saccard is struck by plans developed by his upstairs neighbor, the engineer Georges Hamelin, who dreams of restoring Christianity to the Middle East through great public works: rail lines linking important cities, improved roads and transportation, renovated eastern Mediterranean ports, and fleets of modern ships to move goods around the world.

In addition, Saccard has an intense rivalry with his brother Eugène Rougon, a powerful Cabinet minister who refuses to help him after his bankruptcy and who is promoting a more liberal, less Catholic agenda for the Empire.

Furthermore, Saccard buys several newspapers which serve to maintain the illusion of legitimacy, promote the Banque, excite the public, and attack Rougon.

The novel follows the fortunes of about 20 characters, cutting across all social strata, showing the effects of stock market speculation on rich and poor.

The effects on the characters of L'argent are disastrous, including complete ruin, suicide, and exile, though some of Saccard's syndicate members escape and Gundermann experiences a windfall.

Because the financial world is closely linked with politics, L’argent encompasses many historical events, including: By the end of the novel, the stage is set for the Franco-Prussian War (1870–1871) and the fall of the Second Empire.

All of the descendants of Adelaïde Fouque (Tante Dide), Saccard's grandmother, demonstrate what today would be called obsessive-compulsive behaviors to varying degrees.

In Le docteur Pascal, Zola describes the influence of heredity on Saccard as an "adjection" in which the natures of his avaricious parents are commingled.

In Le docteur Pascal, Maxime is described as prematurely aged, afraid of pleasure and indeed of all life, devoid of emotion, and cold, characteristics introduced in L'argent.

Unlike Jacques Lantier (his second cousin, see La bête humaine), he is unable to control his criminal impulses, and his disappearance into the streets of Paris is no surprise.

In Le docteur Pascal (set in 1872), Zola tells us that Saccard returns to Paris, institutes a newspaper, and is again making piles of money.