Youngest Sister Class.

Lim Da-yeong, the older sister, intercepted the lover of her younger sister, Lim Da-jeong, who is Yoon Tae-jin. As if this were not enough, Da-yeong imprisoned her younger sister in a mental hospital and lived a happy married life with Tae-jin. Then one day, the Da-jeong escapes from the mental hospital and conspires with helper Jo Yong-hwan, who is her ward friend, to exact cruel revenge on her older sister and her former lover. Both Da-yeong and Tae-jin endure Da-jeong's bloody kidnapping, domesticated slave life and harsh punishments. They look for opportunities to escape with a scary plot in their hearts. What will be the ending for these crazy sisters?