William Adamson, Mark Rylance a naturalist, returns to Victorian England after a long expedition to the Amazon, having lost his possessions in a shipwreck. He now depends on his patron, Sir Harold Alabaster, Jeremy Kemp a wealthy Baronet, and is hired to catalog the collector's specimen collection and teach his younger children the natural sciences, assisting their governess, Matty Crompton, Kristin Scott Thomas. The arrangement places William at the crossroads of a glittering, closed world and the rigorous, patient work of science, with the unassuming Matty quietly guiding the classroom and the household.
William becomes enamoured of Eugenia, Patsy Kensit the eldest daughter, who is softly spoken, anxious, and mourning the recent death of her fiancé. Despite his impoverishment, Eugenia accepts his marriage proposal. Although Sir Harold grants his approval, Eugenia's snobbish and spoilt brother Edgar, Douglas Henshall, dislikes William's humble origins. The tension between social expectation and genuine affection threads through the early days of the marriage, as Eugenia navigates a world that both entices and confines her.
Soon after the marriage, Eugenia becomes pregnant. Eugenia's behaviour alternates between coldness, locking William out of her room, and intense sexual passion, hinting at a marriage built as much on secrecy and power as on love. The couple has five children, and she names one son Edgar, in her family's tradition, a detail that grates on William and slowly widens the rift between them. The household hums with routine and curiosity, while William spends long hours with the Alabaster children and Matty, observing an ant colony in the forest, with a view to writing a book that blends meticulous natural observation with a portrait of the people around him.
Returning via the stables from an excursion, William discovers Edgar raping a teenage servant. Edgar tells William that she consented, but the scene is unmistakably coercive and terrifying. William's sense of reality tightens as he confronts a clash between the surface civility of the Alabaster home and the brutal undercurrents it conceals. He forms a fragile bond with Matty, who encourages his scientific pursuits and displays a striking intelligence. The book is successfully published, bringing William a measure of professional and personal validation, even as the personal drama inside the house continues to unfold.
One day, William is summoned from the hunt to the house by a servant who claims Eugenia wants him. Entering the bedroom, he finds Eugenia and Edgar engaging in incestuous sex. Eugenia confesses that this has occurred since childhood and that her fiancé committed suicide as a consequence. She explains that when the revelations began, she was too young to understand, but a change in perspective left her guilt-stricken. In tears, Eugenia admits that she tried to stop, but Edgar's will was too strong. William realises that he has been used to conceal the incest and that the children (who bear no resemblance to him) are Edgar's, a revelation that stains every memory of the marriage.
Matty reveals her knowledge of the affair to William during a Scrabble-like game. Later, she explains that the servants were also aware and arranged for him to discover the truth, a calculated move to expose the secrets that bind them all. Expressing frustration at her life and dependency on the Alabasters, Matty reveals that she has published her own book on the insects and has bought tickets for a ship for the Amazon. William is reluctant; despite his attraction, he feels that the rainforest is unsuitable for a woman. After she assures him of her strength and love for him, William acquiesces, choosing a future that promises both companionship and escape from a painful past.
Before leaving, William meets Eugenia and tells her he intends never to return but will continue to financially support their children. He also promises to keep her secret, for fear of injuring her ailing father, and hopes she may find a way to live with her guilt. William and Matty depart in a coach for Liverpool, eager to begin their new adventure and leave the past behind, stepping into a life shaped by shared curiosity, quiet rebellion, and the promise of a freer future together.
