Twenty-seven-year-old Chihiro Azuma is haunted by memories of his first love Ruka, who committed suicide thirteen years ago. Through a dating app, he meets a high school girl with the same name, creating an unexpected connection.

Seventeen-year-old Ruka Mizuhara uses dating websites to fill the void left by her absent father Naohito. When a sexual video of her leaks online, her carefully managed double life begins to unravel.

Their fates intertwine with Takeru Ishida, a boy suffering family abuse, after Ruka's father kills Takeru's brother Koki to protect her. Consumed by revenge, Takeru murders his mother and takes Ruka hostage at school.

In a climactic confrontation, Chihiro takes a knife wound meant for Takeru's father, embracing both teenagers and assuring them they're not to blame for their circumstances.

One year later, Takeru writes from detention, working toward forgiveness, while Chihiro and Ruka, now engaged, visit his first love's grave—their lives forever changed by one intense, emotion-saturated summer.