Hyun-su and Soo-jin have just gotten married. Seemingly out of nowhere, he starts talking in his sleep. 'Someone’s inside.' From that night forward, whenever he falls asleep, he transforms into someone else with no recollection of the night before. Soo-jin is overwhelmed with anxiety that he’d hurt her family while she sleeps and can barely sleep a wink because of this irrational fear. Despite sleep treatment, Hyun-su’s sleepwalking only intensifies, and she begins to feel that her unborn child may be in danger...