The magic tree house was on a tree with white flowers on the side of a mountain. Nearby was a rushing stream. Suddenly, Jack saw two ninjas standing on rocks near the stream. One of them was tall, and the other short. Both wore black shirts and pants. They also had black scarves around their heads and swords on their backs.

Jack wanted to learn more about ninjas from the books, but before he could do that, the two ninjas had already noticed the tree house and were climbing up the tree.

What would happen to Jack and Annie, who knew nothing about ancient Japan? What were they going to do with these mysterious warriors?