Evelyn is deeply engrossed in her studies of lepidopterology under the tutelage of the older and experienced Cynthia, who regularly delivers lectures about her findings. Their relationship is of a romantic nature, and Evelyn also takes on the role of a maid in Cynthia's household, which comes with stringent behavioral expectations and an unyielding standard of cleanliness. Whenever Evelyn fails to meet Cynthia's rigorous expectations, she faces punishment.
As Cynthia's grip on dominance begins to wane, it becomes increasingly evident that Evelyn is the one orchestrating their dynamic. She meticulously crafts instructions and scripts for their daily play scenes, which the couple reenacts with unwavering regularity. While Evelyn finds these scenarios thrilling and pleasurable, Cynthia merely indulges them to satisfy her partner's desires. In an attempt to surprise Evelyn for her birthday, Cynthia arranges for a carpenter to build a special bed with a drawer underneath, intended as punishment. However, Evelyn is displeased with the lengthy wait time for the bed's completion and ultimately rejects the gift.
This leads Evelyn to assert further control, demanding that Cynthia confine her in a trunk each night as a form of punishment. Cynthia agrees but secretly grapples with her resentment regarding this physical separation. She starts to feel self-conscious about her age, especially after injuring her back while moving the trunk to her bedside. Their relationship begins to fray as Evelyn’s ever-growing demands remain unmet. Tension rises when Cynthia, consumed by jealousy, accuses Evelyn of polishing another lecturer's boots—an act she perceives as a betrayal.
The situation intensifies on Evelyn’s birthday when Cynthia insists that Evelyn bake her own birthday cake, which Cynthia relishes while casually lounging with her feet resting on Evelyn's face. This scene leaves Evelyn feeling uncomfortable, and she calls out her safeword, pinastri, though Cynthia disregards it.
As they attempt to repeat the familiar play scene, Cynthia unexpectedly breaks character and begins to sob. In a tender moment, Evelyn reassures her of her love. Subsequently, the film portrays the destruction of Evelyn's instructions, signifying a shift in their dynamic. The couple then carries the trunk meant for Evelyn’s confinement. The story culminates with Evelyn standing in front of the house, ringing the doorbell, suggesting the relentless cycle of their relationship is about to begin anew.
