Having grown up an only child on his father's large estate in North Wales, Casimir Fleetwood is a young man freshly educated by nature itself. He was largely left to his own pursuits as a child, a detail which serves to complicate his social interactions because he's somewhat awkward and spoiled. When he is sufficiently old enough, he undertakes the customary "grand tour" of the western world in which sons of the British aristocracy often engaged. In Switzerland he meets Monsieur Ruffigny, his father's old friend. While he's traveling, Fleetwood learns that his father has died. Ruffigny escorts him back to Wales, acting as the boy's benefactor for sometime.
Decades pass and when Fleetwood is 45-years-old he marries Mary Macneil. Because of his unusual upbringing he struggles to adapt to the marriage, often mistreating Mary for small offenses. For her part, Mary is an orphan and much younger than Fleetwood. She's in need of a protector. Just before they find out their pregnant, Fleetwood invites his younger cousins Kenrick and Gifford to come live with them, perhaps looking for a distraction from his somewhat challenging home life.
Gifford arrives with his own private intentions. He wants to discredit his brother and insinuate himself into Fleetwood's estate. He begins infecting Fleetwood's subconscious with suspicions about his wife and Kenrick. By the time her pregnancy is discovered, Fleetwood already believes it is the result of an affair with Kenrick. In a strange series of events, Fleetwood finally believes he's found evidence when he reads an anonymous love letter to Kenrick which is in Mary's possession, but she's merely delivering it on behalf of her friend, Louisa. Fleetwood divorces Mary and denounces her to the public, moving to France.
On his way to France, Fleetwood is accosted by thieves who beat him. By chance, Louisa's father passes by and explains to Fleetwood the mistake he made regarding Mary and Kenrick. He also points the blame at Gifford, offering some persuasive evidence of Louisa's against the man's character and adding that Kenrick is the one who rescued Fleetwood from the robbers. Repenting from his haste, Fleetwood forgives the couple. He writes a new will which leaves most of his estate to his wife and son as well as granting a portion to Kenrick. Then he moves to the Pyrenees, having long ago lost his taste for marriage. In the end Gifford is executed for highway robbery, and Kenrick and Louisa get married to each other.
