The novel opens with the action of the plot already underway. The reddleman Diggory Venn rides onto the heath with Thomasin Yeobright in the back of his wagon: her marriage to Damon Wildeve was delayed by an error in the marriage certificate, and Thomasin collapsed. We soon learn that Wildeve orchestrated the error himself. He is infatuated with Eustacia Vye, and is, at least to some extent, using Thomasin as a device to make Eustacia jealous. When Venn learns of the romance between Eustacia and Wildeve, his own love for Thomasin induces him to intervene on her behalf, which he will continue to do throughout the novel. But Venn's attempts to persuade Eustacia to allow Wildeve to marry Thomasin, like his own marriage proposal to Thomasin, are unsuccessful.

Into this confused tangle of lovers comes Clym Yeobright, Thomasin's cousin and the son of the strong-willed widow Mrs. Yeobright, who also serves as a guardian to Thomasin. Eustacia sees in the urbane Clym an escape from the hated heath. Even before she meets him, Eustacia convinces herself to fall in love with Clym, breaking off her romance with Wildeve, who then marries Thomasin. Chance and Eustacia's machinations bring Clym and her together, and they begin a courtship that will eventually end in their marriage, despite the strong objections of Mrs. Yeobright. Once Wildeve hears of Eustacia's marriage, he again begins to desire her, although he is already married to Thomasin.