"Long ago, after the Evil Samurai Warlord was banished the first time, an era of peace and tranquility descended upon the empire for more than 300 years. The dark castles he had built were abandoned and slowly fell into ruin and the villagers across the countryside no longer lived in fear. As the years went by, the secret of the powerful Kuji magic was hidden away in ancient shrines, and the people of the villages and towns went back to their daily lives.
Then one day, something changed. Terrible storms began to brew over the mountain on the distant horizon. Floods and earthquakes began to appear in the flatlands. At Ninja Mountain, The Ninja Master could see that something was terribly wrong in the world. Mysterious figures were seen hunting and caging animals for unknown reasons. The truth was that the Evil Samurai Warlord had returned, using the forbidden Kuji magic to transform innocent animals into mindless samurai warriors to use to recapture his dark castles and take over the world. But, by using the Kuji magic for evil purposes, the Evil Samurai Warlord was once again upsetting the balance of nature, bring the wrath of the gods down upon the earth in the form of ravaging storms. The larger his armies grew, the worse the storms became.
The Ninja Master knew none of this yet... though he knew that something must be done and made it his unbreakable oath. He sent his best ninja to discover the truth of the storms, but no word returned from the ninja. One by one, he sent another, but again and again... nothing. Until he was left with only two left, Hiro and Futo. The Ninja Master was hesitant to send his last two, but he knew the fate of the world would lay in their hands."
Source: Wikipedia, "Mini_Ninjas," available under the CC-BY-SA License.