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大切なことはすべて君が教えてくれた
Shuji is a young teacher whose confidence and honesty has made him popular with the students. He is engaged to Natsumi, also a teacher at the same high school. At the morning of the new semester, Shuji wakes up with a hangover and no memory of the night before, and a strange young woman in his bed. He feels terrible about possibility of having cheated on his fiancée, but when it turns out the girl is a student in his class Shuji is wrecked with guilt. Things go from bad to worse when the girl, Hikari, makes it clear that she's in love with him and wants to break him and Natsumi apart. Shuji's honest nature makes him want to come clean to everyone about what has happened, but before he has the chance, the secret is out. As his life starts crumbling around him, Shuji still decides to try and save Hikari from herself.
Shuji wakes up in the morning to a very troublesome situation. He doesn't even begin to comprehend how his life is about to change.
As Shuji and Natsumi continue their wedding plans, Saeki keeps trying to interfere. Natsumi, feeling uneasy with her around, finally gives to the pressure and confronts the girl. In the meantime, Shuji has to deal with a problem between two of his female students who are known to be best friends.
Natsumi and Shuji attempt to deal with the problem, but things only seem to get worse and cause a conflict with Shuji's sense of mission as a teacher. Saeki opens up to him about something that happened in her past.
Students accidentally learn a shocking fact about Shuji.
As Shuji suffers the consequences of his actions, Saeki decides to speak up.
Shuji is forced to leave the school and Natsumi for a while. When he returns, he questions his motivations and feels out of place in the school. Also, Natsumi has more news in store for him.
Shuji and Natsumi cancel their wedding. Shuji begins to spend more time with Saeki, while a chance appears for Natsumi to help her move on with her life.
Shuji's mistakes have had very bad effect on the school. He is presented with a choice and finally comes to take a more decisive stance for himself.
As the school year draws to a close, Shuji prepares to move on. During the gradution ceremony, he and Natsumi schedule to meet the next day. However, on his way home he bumps into Saeki.
Shuji says goodbye to Saeki and tries to get a hold of Natsumi.
