Manga by author Yuuki Masami

Meet Tsutomu & Birdy - a boy and a girl from opposite ends of the galaxy with absolutely nothing in common. Tsutomu was an ordinary high school student. Birdy a maverick Space Federation Officer armed with superhuman strength and - as Tsutomu soon discovers- a devastating energy blast. Now they have one thing in common: Their body. While apprehending an alien felon hiding out on Earth. Birdy accidentally blasted Tsutomu- killing him instantly. Now, Tsutomu must "time-share" his body with Birdy. And when she isnot yanking him around like a marionette, he transforming into a girl in full view of his friends and family!

After two years of being dormant inside Tsutomu Senkawa, an accidental encounter with their old nemesis violently wakes Birdy up. With Birdy kicking evil's ass once again, the dark secrets of the experiments conducted by the "Agni" corporation are slowly revealed. They seem to be dabbling with human body enhancement, creating what they call "beast men," to combat the alien invasions related to Birdy.

The manga tells the story of what happens up until the birth of Astro Boy and is considered an "episode zero" story.

Kuze Shunpei, a high school student, is on spring break going to Hokkaidou when he loses his wallet. Stranded in the mountains with no money and his motorcycle having run out of gas, he passes out in the snow on the side of the road. When he wakes up he finds that he's been saved and brought to the Watarai farm, a horse ranch that breeds race horses. There he meets a host of various people, ranging from stable hands to the owner and his family (which includes four daughters).Shunpei is completely unsuited for the farm life, and his city upbringing contrasts sharply with the hard labor required to help the farm out. Yet despite all this, he tries his best to become accepted. However, he constantly gets into arguments with Hibiki (the second daughter), who views him as useless. Things come to a head when Shunpei scares one of the horses, who throws Hibiki (Hibiki is a rider). Ashamed, Shunpei flees back to Tokyo for the rest of the spring break. But back in Tokyo, Shunpei begins to long for the simple farm life. Will Shunpei ever be able to return and prove himself to be worthy of being a member of the farm?