Wen Zhiyi, a junior designer, takes in a temperamental black stray cat on a rainy night, unaware that every nightfall it transforms into a man wearing an apron, who cleans her apartment, makes soup, and revises her designs. When her insomnia leads her to open the kitchen door and discover the true identity of this 'snail spirit guest,' their mismatched life together begins: cat cuddles by day and living together by night. Facing workplace bullying, the black cat's picky habits unexpectedly become Wen Zhiyi's secret weapon in winning the lead designer position for a home furnishings exhibition. But as its demonic aura begins to leak, attracting unwanted attention from the outside world, this proud and arrogant great demon must not only hurry back to the apartment before sunset to protect his identity, but also confront a difficult choice between repaying her kindness and following his growing feelings: is he willing to remain in the human world for her?