A ninja is looking for a young child to take his place as the number one ninja to go against a mysterious creature that as been defeated each year and he needs a young strong child to make sure the creature does not take over the city.
After the ninja talked to the teen about the creature and how it could take over the city if the ninja does not get the teen help. So the teen thought about it and he was cool with the idea to help the ninja fight against the creature
So the ninja had took the young teenager with him to go train him to go against the mysterious creature
The ninja has finished his training with the teenager and that is when the myst