Twenty-year-old Ki-chan lives with his father until his father brings home a new, significantly younger wife named Yoon-seo. While Yoon-seo attempts to settle into the household and act as a friendly maternal figure, Ki-chan becomes fixated on her sexually and pursues her as a romantic prospect. Despite Yoon-seo's initial resistance, the two enter a passionate, secret extramarital affair behind the father's back. When Ki-chan's friend Min-jeong discovers their clandestine liaison, Ki-chan and Yoon-seo are forced into a secret pact to maintain her silence and protect their taboo relationship.