Planning to go away to Japan with Grandpa is not as easy as Rocky, Colt, Tum-Tum had hoped. After expressing their different views on how they want to spend their vacation, Mori becomes concerned that his grandchildren are growing apart, both from himself and from each other. Tum-Tum seems to be the only one interested in traveling to Japan, and even then only hopes to go and watch Sumo wrestlers due to the amount of food they are able to consume every day, while Mori intends to return a dagger to the Grand Master at a martial arts tournament that he had won 50 years prior from a boy named Koga. Meanwhile, in Japan a valuable sword goes missing when a thief dressed as a ninja steals it and escapes via hang glider.
Back in the United States, the boys experience the pains of growing up at a baseball game; Colt has become impatient and short tempered, starting fights with bullies on the opposing team, while Rocky is too distracted by hottie Lisa Di Marino to pitch properly, and Tum Tum's obsession with food causes constant delays in the game. When their troubles come to a head, both teams get into a massive brawl that forces the umpire to delay the game a week, putting a nail in Mori's plans to take the boys to Japan. When they take him to the airport though, a sticker falls off Tum Tum's bag and Sam, the boys' father, grabs it by mistake. The boys later discover intruders at Mori's cabin and fend them off, not realizing they were the platinum haired nephew of Koga, Glam and his friends hired to steal the dagger. That night after Mori calls and says a fender bender 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 to return the dagger to the Grand Master. Koga punishes them for failing to deliver the dagger three times and formulates his own plan to get it. At the martial arts tournament, Colt dresses as a competitor, but is beaten and by a girl named Miyo (Caroline Junko King) who has a passion for baseball, but not the skill. They make a deal where she teaches them to further martial arts and they teach her baseball, which works out beneficial for everyone, and Rocky develops feelings for her. Meanwhile, Mori is kidnapped by Koga's assistant.
The Grand Master arrives at Miyo's house, but unlike before he can speak English, and he wears shoes which rouses Tum-Tum's suspicions. Miyo uncovers the deception and alerts them and they all fight off Koga's men before they too are captured. With the sword and dagger in hand, Koga uses the boys as leverage to make Mori tell him where the fabled cave of gold is hidden. The kids escape after them on hang gliders and reunite with Mori before Koga is able to shoot him. Using what he'd learned for training, 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 is grateful to Mori, Miyo and the boys, Koga made peace with Mori, his grandkids and Miyo, who realize that they can still make the delayed championship baseball game. At the game, Rocky manages to almost strike out the batter who hits a pop fly to a new player that catches it in her hat, revealing Miyo had joined the team. In the next inning, Rocky and Tum-Tum make it on base and Colt strikes a home run to win the championship. To make it seem fair, Colt allows the opposing team pitcher to pick one of the three of them to fight and they pick Miyo, despite Tum-Tum warning them that "she's just a girl" they try to attack her before she beats the group up and the screen goes dark.