Set over a day in 1924, the story follows Jane Fairchild (Odessa Young), a maid in the Niven household, who has the day off to celebrate Mothering Sunday while Mr and Mrs Niven (Colin Firth and Olivia Colman) attend a lunch to celebrate the engagement of their neighbor's only remaining son, Paul (Josh O'Connor), to Emma Hobday. Jane rejoices at her freedom on an unseasonably hot, beautiful spring day. But, she has no mother to go to.

For almost seven years she has – joyfully and without shame – been Paul’s lover. Like the Nivens, Paul belongs to England’s old money aristocracy, whereas Jane was orphaned at birth. With the house conveniently empty, they can finally meet in Paul’s bedroom for the first time. Today will be their last as lovers. It is also the day that will mark the beginning of Jane’s transformation as the story unfolds through the hours of clandestine passion.