William Adamson is a naturalist who has been traveling; he returns to Victorian England and stays with his benefactor Sir Harold Alabaster because he was shipwrecked during his trip to the Amazon and lost all his possessions. Alabaster gives him a job; he is in charge of cataloguing his specimen collection, and he also teaches his children about natural sciences. He develops a soft spot for Eugenia, Harold's oldest daughter. She is in mourning for her late fiance, and is rather emotionally vulnerable. Because of this she accepts Adamson's proposal of marriage. Her father is happy about their upcoming union, but her brother is a snob and looks down on Adamson because of his financial status.
Eugenia becomes pregnant very quickly after they are married. She has intense mood swings one minute cold and frosty, the next sexually wild and avaricious. The couple go on to have five children together but they do not remain emotionally close or intimate. William spends most of his time in the forest with the children looking at bugs and researching a book he is thinking of writing. It is when he is returning from one such excursion that he sees Edgar, his brother in law, raping a servant out in the stables. The servant is a teenager. Edgar claims that she gave her consent but the girl looks terrified and William finds this hard to believe.
On another occasion, a servant calls him back to the house saying that Eugenia has asked for him, but when he goes into the bedroom to find her he bursts in on Eugenia and her brother having sex. Eugenia tells him that they have been having sex since they were children; at first she was too young to understand why it was wrong and what the consequences might be; when she began to see the situation the way in which her fiance had seen it, she began to realize what she was doing. She tried to end their sexual relationship but Edgar does not allow her to do so. When her fiance found out he killed himself. All at once William realizes that he has been used to cover up the incestuous relationship. He also realizes that the children he believed to be his own, yet who bear such little resemblance to him, are Edgar's children.
Matty, the children's governess and William become ever closer. She tells William that she knew about the affair all along. She tells him the servants all knew as well and they were behind his finding out. Matty has purchased a ticket for herself for a trip to the Amazon and she has also purchased one for him as well. William is very tempted, but he is a chauvinist and feels that the Amazon is no place for a woman. However, Matty summons up the courage to tell him that she is in love with him and he agrees to accompany her.
Before they leave for the Amazon, William sees Eugenia for the last time. He tells her that he will keep her secret but not for her good; he does not want to upset her father, who is not well. He leaves with Matty, and they embark on the rest of their lives together.
