"Unnie, we're each other's only 'sister', aren't we?"

High school student Ryu So-hee, who was very sick during her childhood but has now recovered to live a normal life, has a sick younger sister named Ryu Ah-young. Although Ah-young is deeply attached to her older sister, So-hee begins to find Ah-young's affection burdensome. As their relationship begins to drift apart, the two sisters grow increasingly uncomfortable around each other. A cruel mystery thriller about two sisters, unfolding beneath a hidden family secret!