Hisao Nakai is an ordinary high school student and has been studying there for the last year. One day he finds a letter from an unknown sender in a locker and arrives at the specified meeting place. The author of the letter turns out to be a girl who loves him, but when talking to her, he loses consciousness. When he wakes up in the hospital, he learns that the reason for his fainting is an unexpectedly manifested heart disease - arrhythmia. After a long treatment in the hospital for arrhythmia, he is forced to transfer to the Yamaku school, where, despite the difficulties, he finds friends and love.