Each volume depicts a different heroine.
 
Volume 1: Main character of the story is Tachibana Tsubomi (立花つぼみ), a fifth grader dealing with issues such as her mother's pregnancy, her first period, and the strange feelings she has started to get when she has been around boys. 
Volume 2: Main character of the story is Kusakabe Tsubomi (草壁つぼみ), a fifth grader, who encounters an alien wanting to do research of earth. It turns out he has picked Tsubomi to be his research subject. 
Volume 3: Shiina Tsubomi (椎名 つぼみ) 
Volume 4: Katanashi Tsubomi (小鳥遊 つぼみ) 
Volume 5: Aizaki Tsubomi (相崎 つぼみ) 
Volume 6: Nike Tsubomi (仁家 つぼみ) 
Volume 7: Yagi Tsubomi (八木 つぼみ) 
Volume 8: Honda Tsubomi (本田 つぼみ) 
 
Note: Won the Shogakukan Manga Award for children's manga in 2009.

Tsubomi Tachibana is a fifth grader dealing with rather sensitive issues such as her mother's pregnancy, her first period and the strange feelings she has started to get when she has been around boys. 
(Source: Anime News Network)

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