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A playboy's life changes after he met and slept with a girl who hasn't turned 18 yet.

At some point, Manato Irie (Hideaki Takizawa) began to withdraw himself from the world. Even when picked on, it's not the real Irie, but an act. However one day he meets a strange girl - Yui Misawa (Kyoko Fukada). Yui, who finds the tedium of everyday life unbearable, teases Manato with her almost too upfront and honest manner. Alone and afraid Manato wavers but finds himself drawn to her. But his feelings for Yui are doomed to be unrequited, when he discovers that Yui is none other than his new step sister. While he struggles with his feelings for Yui, Manato begins to receive blue love letters in his shoe locker at school. He eventually discovers that they were left there by his next-door neighbour, Haruka (Uchiyama Rina). Meanwhile, Yui finds herself falling for senpai Saeki (Kubozuka Yosuke), who in turn is having an affair with his teacher, Asami-sensei (Ishida Yuriko).