On a warm spring day in 1924, housemaid and foundling Jane Fairchild (Odessa Young) finds herself alone on Mother's Day. Her employers, Mr. and Mrs. Niven (Colin Firth and Olivia Colman), are out and she has the rare chance to spend quality time with her secret lover. Paul (Josh O'Connor) is the boy from the manor house nearby, Jane's long-term love despite the fact that he's engaged to be married to another woman, a childhood friend, and the daughter of his parent's friends. But events that neither can foresee will change the course of Jane's life forever.