Midori Makibaō is a small white mule. Compared to the other thoroughbred horses, his physical size comparable to a donkey with wide nostrils. However, with guts and speed as his weapons, Makibaō wins in every big race.
At the beginning of his career, Makibaō has trouble making his debut as a racehorse. But he has reasons to overcome his difficulties; a lifelong rival horse called Superhorse Cascade, that Makibaō needs to defeat. Plus, he is determined to win back his mother, Midori, a horse that had been taken away to pay debts.
There are many hardships that Makibaō has to overcome, but he thrives in a series of races to become a great racehorse.