After nearly drowning, Yein was saved by Jungkook. She pretends that she can't remember anything. Jungkook's mom thought that she was Jungmi, her lost daughter. Jungkook saw happiness in his mom's face as soon as she saw the girl and because of that, he made a deal with Yein to pretend that she is his younger sister. Can Yein adapt to her new life? As siblings, will love still blossom between them?