The Kidnapping Day follows Kim Myeong-jun, a desperate, soft-hearted man who decides to kidnap an 11-year-old girl named Ro-hee to pay for his daughter's hospital bills. However, the plan goes awry when Ro-hee loses her memory in an accident, leading Myeong-jun to pretend to be her father while they live together. As they navigate their bizarre living situation, they form an unexpected bond that transcends their circumstances. As the story unfolds, it is revealed that Ro-hee is a genius child from a wealthy family, and her kidnapping is tied to a much darker conspiracy involving her parents' mysterious deaths. Myeong-jun and Ro-hee must work together to uncover the truth behind the kidnapping and the sinister secrets hidden within her family, all while being pursued by the police and dangerous individuals who want to keep the truth buried.