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.
In marrying Eustacia, Clym distances himself from his mother. Yet distance soon begins to grow between the newlyweds as well. Eustacia's dreams of moving to Paris are rejected by Clym, who wants to start a school in his native country. Wildeve inherits a substantial fortune, and he and the unhappy Eustacia once again begin to spend time together: first at a country dance, where they are seen by the omnipresent observer Diggory Venn, and then later when Wildeve visits Eustacia at home while Clym is asleep. During this visit, Mrs. Yeobright knocks at the door; she has come hoping for a reconciliation with the couple. Eustacia, however, in her confusion and fear at being discovered with Wildeve, does not allow Mrs. Yeobright to enter the house: heart-broken and feeling rejected by her son, she succumbs to heat and snakebite on the walk home, and dies.
Clym blames himself for the death of his mother; he and Eustacia separate when he learns of the role that Eustacia played in Mrs. Yeobright's death, and of her continued relations with Wildeve. Eustacia plans an escape from the heath, and Wildeve agrees to help her. On a stormy night, the action comes to a climax: on her way to meet Wildeve, Eustacia drowns. Trying to save her, Wildeve drowns as well. Only through heroic efforts does Diggory Venn save Clym from the same fate. The last part of the novel sees the growth of an affectionate relationship, and an eventual marriage, between Thomasin and Diggory. Clym, much reduced by his travails and by weak eyesight brought on by overly arduous studies, becomes a wandering preacher, taken only half-seriously by the locals.
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