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Koga and Mori participates at the martial arts tournament where Mori won after his battle against Koga in finals. The Grand Master give Mori the dagger prize. Eventually, Koga attempts to steal the dagger from Mori, ends up getting a scar before running away.
During the 50 years prior, Koga leads the Yakuzas organization with his bodyguard Ishikawa. Meanwhile, Koga breaks inside the Museum of Japan dressed as Ninja to steal a valuable sword from the display before setting off the alarm. After fighting through several security guards, Koga escapes via hang glider and returns to his headquarter as he swore revenge on Mori
Back in the United States after the baseball game delay from the massive brawl with the opposing team Mustangs, Koga calls his platinum haired nephew, Glam and his friends, Vinnie and Slam at the nightclub (while they're performing their own band) and bribes them to steal the dagger from Mori's cabin for the money, which he accepts. After the boys decided to remain at home and Mori leaving his cabin, Glam and his friends sneaking onto Mori's cabin until the Boys discover this and fends them off. Later that Night, Glam and his friends called Koga about their first attempt failure and he sternly demands them to get the dagger once again. After Glam and his friends follows Mori and manage to steal his bag, Koga exercising on the treadmill with Ishikawa while Glam and his friends return to present the bag to him. Unfortunately, Koga realize that Glam and his friends stole the wrong bag (because to Sam, boys' father, unknowingly giving Mori the bag that was actually belongs to Tum Tum after the sticker falls off from it). Koga punishes Glam by having Ishikawa running on the treadmill and sternly giving his nephew and his friends an another chances to retrieve the dagger once again.
That evening after Mori calls and saying about a crash that put him in the hospital, Tum-Tum discovers the bags were mixed up and the boys make arrangements without their parents' knowledge to travel to Japan using a recorded call from Mori. When they arrive, they reveal to him that they have the dagger while Glam and his friends, who were responsible for the fender bender, record Mori's instructions with wiretap device to return the dagger to the Grand Master during the Tournament.
After Glam and his friends return with the tape about Mori's instructions, Koga listens to it on the boombox and formulates his own plan to get it before punishes them for failing to deliver the dagger the three times since they brought him was Mori's voice record tape by having Ishikawa throwing them into the swimming pool. During the following day, Koga and his men arrives at the Grand Master after he's knocked unconscious from the tea and they tied him up and stole his clothes to impersonate him. Koga (as the Grand Master) talks with the boys, but unlike before he can speak English, and he wears shoes which rouses Tum-Tum's suspicions. Miyo uncovers the deception after she finds the real grand master at the storage and alerts them about it. The boys and Miyo runs away as Koga orders his men and ninjas to chase after them. After they all fight off Koga's men and ninjas before they're captured at the secret passage. With the sword and dagger in hand, Koga takes the boys and Miyo to his headquarter and locks them in their cell. Knowing Mori has the knowledge, Koga meets Glam and his friends at the training room and orders them to get Mori at the hospital while Ishikawa awaits for them. After Mori dealt with Glam and his friends at the hospital, he is kidnapped by Ishikawa and driven to Koga's headquarter.
Koga intimidates Mori to tell him where the fabled cave of gold is hidden, which he reveals that it was beneath Castle Hikone. The kids escape the organization with the hang gilders after they fought through several of Koga's men, ninjas and sumo wrestlers. When the kids fight against Ishikawa and some of the ninjas, they reunite with Mori after he and Koga found the cave of gold before Koga is able to shoot him. Using what he'd learned for training about focusing on the target, Colt blocks Koga's gun with a ball bearing and it backfires, causing a cave-in. As they escape, Koga leaves the riches behind to save his own life and even throwing the sword inside the cave, sealing it for good. Afterwards, Koga halts his ninjas from their attacks, he is grateful to Mori, Miyo and the boys and made peace with them before leaving them in to renounce his actions.