Jack is the main villain of "3 Ninjas: Knuckle Up".
Rocky, Colt, Tum-Tum defend "Truth, Justice and the American Way", once more - this time, protecting a Native American village and the rest of society against a Toxic Waste Company.
During a summer the boys are staying with Grandpa Mori, the boys encounter a group assaulting a girl named Jo at a pizza parlour, after fending off the men they are praised for their martial arts techniques, which gives them big heads. Despite their efforts, they are put to work by Mori and the pizza owner to work off damages, Mori tries to teach them a lesson in humility but the reference of a flower blooming goes over their heads. Jo comes to the boys later and explains that the men are under the employ of Jack Harding, an industrialist who is illegally dumping toxic contents into the reserve, without proof, they can do nothing. Jo's father had gone to investigate but had not returned. Colt, who is seemingly attracted to Jo says that they will help, and they mount an escape plan for her father that night, which is successful. They spend the night celebrating with the tribe and getting thanks for helping them. Jo's father appeals for a court date with significant evidence to put Jack out of business for good, undeterred, Jack arranges to have Jo kidnapped and convince her father to falsify his evidence, which he has no other choice.
Rocky and the others get information to where Jo is being held and drive out to free her and return before the court case is dismissed and all of her father's hard work accounts for nothing. After working through a small band of armed men, they find Jo and return her to the court house just before her father turns the real evidence over to Jack he admits his mistake and hands the evidence to the judge who deems the case and shuts down the company producing the waste. Jo looks around for the 'heroes' of the day, but they are nowhere to be found. Rocky realizes Mori's words of what a flower says when it blooms and they return to Mori, sharing that they say that a flower doesn't show off its beauty, it's there for someone to find, and Mori is ecstatic at the realization.