Gaon has been living in isolation for years at the Baekam Island mansion, caring for her sick younger sister. One day, her uncle brings home strangers: a doctor to treat her sister and the doctor's nephew, Cha Taekyung. "My name is Cha Taekyung." Though his first impression was unappealing, Gaon had no choice. An uncomfortable coexistence with the strangers begins. However, strange things start happening around the mansion after their arrival, and Taekyung's actions constantly rub Gaon the wrong way. To make matters worse, this man is not easy to deal with. "Have you ever thought about it? That you might be losing your mind." "...What do you mean?" "You seem more sensitive than Baek Gayoung. It looks like the older sister needs treatment, not the younger one." "I don't want to hear that from a real lunatic." Amidst this, Gaon gets entangled in yet another bizarre incident alongside Taekyung, and she eventually joins hands with him to catch the culprit. As the secrets long hidden on the island slowly come to light, Gaon begins to harbor deep suspicions... What is the real reason this man came to Baekam Island?