Cheerful high school girl Ha-na (Lim Ju-eun) is always the first to come out and protect her introverted twin sister Doo-na (Park Ji-yeon), who has a slight limp. After Doo-na es away in a fire, Ha-na finds that her body no longer completely belongs to herself. Doo-na's angry spirit now lives in Ha-na, giving her special powers and monstrous strength. Genius criminal psychologist Shin Ryu (Lee Seo-jin) is an expert profiler who is determined to see justice served. When he learns about Ha-na's abilities, he uses her powers to eliminate criminals that are above the law, and to plan his revenge on the man (Kim Kap-soo) who helped his family's killer go free.