In 1865, in rural England, Katherine (Florence Pugh) is in an unmarried marriage with an older man named Alexander Lester (Paul Hilton). They live in a country villa of Alexander’s father, Boris (Christopher Fairbank). Katherine is forced to maintain a strict program and is not allowed to leave the house. Boris is fighting for not giving Alexander a boy, but the latter does not show any physical interest in her.
One day, both Boris and Alexander are going to leave home with various businesses, leaving Katherine alone with housekeeper Anna (Naomi Ackie). For the first time since she can remember, Katherine is free to explore the area to get rid of her boredom. Exploring the area, she observes some men who work there and feels attracted to one of them, Sebastian (Cosmo Jarvis), with whom he does so “to happen” the next day.
It does not take long and Sebastian visits Katherine at home. Although initially hesitant, she succumbs to his charm, and the two of them make love, and then start a lasting adventure. Anna informs the local priest about what’s going on, and he’s trying to warn Katherine, but she does not take him into account. After a while, Boris returns home. Being informed of Katherine’s adventure with Sebastian, he beats Sebastian and closes him in a cell, threatening Katherine when she asks him to release him. Soon she takes revenge on Boris in an unexpected way.