The novel begins in Paris in 1819. Eugene de Rastignac is a young law student who lives at a boarding house along with a number of other boarders, including an elderly man informally known as "Pere" Goriot. Although Goriot was once extremely wealthy, he has spent most of his fortune on his two daughters, Anastasie de Restaud and Delphine de Nucingen, and his poverty makes him a subject of mockery amongst the other boarders. As Rastignac observes Parisian society, he becomes increasingly obsessed with the idea of achieving wealth. His one connection in high society is a wealthy distant relative named Vicountess de Beauséant. Through Beauseant's connections, Rastignac meets and admires Anastasie, but he only learns of her connection to Goriot after he has inadvertently offended her.
Rejected by Anastasie, Rastignac begins to pursue Delphine, hoping to use the rivalry between the sisters to his advantage. His need to maintain appearances and social decorum begins to sink him into debt, especially after he falls in love with Delphine. As Rastignac grows increasingly desperate for money, he falls under the sway of his fellow boarder, a mysterious and sinister figure named Vautrin. Vautrin suggests a plan: living at the boarding house is a young woman named Victorine de Taillefer. She is the daughter of a wealthy man who has disinherited and disowned her, and Vautrin suggests that Rastignac woo and marry her. He will then arrange for the death of her brother so that she will become her father's sole heir, and she and Rastignac will become very wealthy. Rastignac begins to court Victorine, even while he remains involved with Delphine and with Goriot's attempts to get her out of her unhappy marriage.
Meanwhile, police forces are closing in on Vautrin, who is actually a notorious criminal. Shortly after Victorine's brother is killed as a result of Vautrin's scheme, Vautrin is arrested. Rastignac is now free to devote himself to Delphine, who separates from her husband and moves into an apartment which her father is paying for. However, her husband's financial circumstances make the situation more complicated, and Anastasie is also seeking her father's help because she needs money to pay the debts of her lover. The quarreling between the two sisters, and their selfish demands on their father lead to Goriot falling seriously ill.
As Goriot lays on his deathbed, Rastignac nurses him with the help of his friend, a medical student named Bianchon. Delphine and Anastasie are too selfish and preoccupied to visit their father until it is too late, and they also do not contribute to the costs of the funeral. Despite his selfless generosity, Goriot is buried in a pauper's grave at a funeral attended by no one. Hardened by the experience, Rastignac vows to become cold and calculating in his aims to make his mark on Parisian society.