The police consider it a tragic accident that Yuko Moriguchi's 4-year-old daughter drowned in the school swimming pool. However, Yuko, a teacher at the school, knows that two of her students are responsible for Manami's death, and she does not intend to let the murderers get away with it. The day before the holidays, she informs her class that she wants to give them one last lesson. However, the deviously devised revenge plan spirals out of her control, setting off a deadly drama from which no one will emerge unscathed.