Miss Night and Day introduces a young job seeker who suddenly experiences a perplexing transformation: she becomes a 50-year-old woman by day and reverts to her 20s as night falls. Viewing this bizarre phenomenon as a unique chance, she leverages her mature daytime persona as a highly capable intern with an energetic mindset. She excels effortlessly at work, charming her colleagues and skillfully navigating workplace challenges, even while eager to clock out on time. Her unusual dual life becomes further complicated when a brilliant and relentless prosecutor from the narcotics unit finds himself inextricably linked to her. A driven workaholic, the prosecutor becomes entangled with both the mysterious daytime intern and her younger self at night, leading to an intriguing and challenging dynamic for everyone involved.