Gwendolyn Harleth is the oldest daughter of the widow, Fanny Davilow. She is also the favorite and accustomed to getting her way. As a young adult, she moves to an estate called Offendene with her mother and four sisters in order for the family to be near Mrs. Davilow's sister and her family, the Reverend and Mrs. Gascoigne. There, Gwendolyn soon attracts the attention of her cousin, Rex, who pledges his love for her. Gwendolyn refuses his offer but is soon courted by the heir to the baronetcy, Henry Mallinger Grandcourt. She is considering accepting the offer of marriage she is certain will be forthcoming but learns that Grandcourt has a mistress and several illegitimate children. The mistress believes that Grandcourt should do right by his illegitimate family and Gwendolyn goes away to visit with friends at a resort called Leubronn. There she meets a young man named Daniel Deronda.

Daniel is the ward of Grandcourt's uncle, Sir Hugo Mallinger. Daniel and Gwendolyn are quickly drawn to each other, though they aren't formally introduced on this first meeting. Daniel is a kind man and goes out of his way to help others. One day he sees a young lady, obviously in distress and obviously about to drown herself. He intervenes and becomes her protector. She's a Jewish woman named Mirah and she and Daniel eventually marry. Daniel has grown up without knowing of his parentage but is eventually contacted by his mother. She tells him that he is also Jewish, that she'd handed him over to Sir Hugo to raise, partly in order to save him from being ostracized as a Jew but also because she was an actress and didn't want a child. Daniel comes to terms with his mother, though she tells him that she has no plans for them to meet again or get to know each other.

Gwendolyn, meanwhile, falls on financial difficulty when her mother loses her fortune. Gwendolyn decides to marry Grandcourt anyway, reassuring herself that, as his wife, she can sway him to do right by his illegitimate children. She soon learns that he is controlling and she has no sway with him at all. He chastises her and she is soon miserable though she hides it, fearing that people will pity her. The couple are traveling in the Mediterranean when Grandcourt insists that he and Gwendolyn go for a sail. They are alone on the ocean when Grandcourt is knocked from the boat and drowns. Daniel happens to be staying in the same hotel, having just met with his mother, when the drowning occurs. Gwendolyn tells him that she's to blame for Grandcourt's death, but it seems she feels guilty for wishing him dead and that it actually was an accident.

Gwendolyn returns to her mother's home and Daniel makes plans to marry Mirah. Gwendolyn tells Daniel that she's a better person for having known him and Daniel begins his new life, living openly as a Jew.