Explores what's happening in our bodies when we are happy. Studies the chemical reaction going on in the brain.

Twelve-year-old Ann Uekusa and her mother move to the small town of Shimane, where Ann's grandparents live and everyone seems to be familiar with everyone else. Ann soon becomes friends with Daigo, a young boy who lives in Shimane, as well as Fuji and Shika, Fuji's younger sister and her best friend in Shimane. When Ann's mother, Miwako, commits suicide, Daigo helps Ann cope with the sudden change in her life. They fall in love, but Ann's life can only become more complicated when her estranged father suddenly arrives from Tokyo. After greatly contemplating going to Tokyo to live with her father after he asks her, she decides to stay. Ann tells Daigo about her decision but makes her go anyway. Ann and Fuji both live in Tokyo but hardly ever talk because Fuji goes to an elite high school, K High. Ann met back up with her friends she had before moving to Shimane and they continue their friendships. In Tokyo, she ends up becoming closer to Fuji, which causes conflict between her and Daigo.In Shimane, Shika learns the truth about her parentage and in her confusion, she begins to fall in love with Daigo. When Ann returns to Shimane, Shika upsets Ann's fragile emotional state so that Ann and Daigo will break up. In the end, Ann breaks up with Daigo for fear that her own sadness will weigh him down. Ann returns to Tokyo and starts a relationship with Fuji; however, because she does not love him, the relationship does not last. Daigo refuses Shika and rumors begin to circulate that he is interested in his middle school classmate Ayumu, who has been helping him studying to get into university and become a teacher.Nearing graduation, Ann learns that her father and his friend Kaede Kuroki have been dating and she is pregnant with Ann's half-sister. Ann's father and Kaede marry after being encouraged by Ann and Ann's grandmother. Fuji, now aware that Shika was the child of his mother's affair rather than himself, supports her decision to studying abroad in Vancouver so that she can learn not to rely on others. Daigo and Ann make it to the post-secondary institutions of their choice while Fuji must repeat his senior year after running away from home.After attending her middle school reunion, Ann spends some time with Daigo, where he tells her to find her own happiness. Six years later, Ann meets a young businessman named Keiichiro Sakura and they become engaged shortly after. After Ann learns that he only uses people for his own gain, she confronts him about his insensitivity, but he is disappointed in her and breaks off the engagement. Almost a week afterward, she takes a train to Okayama, where Daigo teaches elementary school. Ann's grandmother, meanwhile, believes Ann will suffer the same fate as her mother. After a failed suicide attempt on the beach, Ann is taken to the hospital and later meets Daigo, who asks her to marry him. She says yes and at the very end the two are shown to be happily married with an infant son named Ryo. Meanwhile, Fuji decides to marry his cousin Mariko (against his family's wishes) and Shiika decides to work for her uncle in America.