In this heartfelt coming-of-age adventure, we follow the journey of three sibling ninjas: Rocky, a 13-year-old aspiring martial artist; Colt, an impulsive 12-year-old; and Tum-Tum, a clever 7-year-old grappling with the complexities of growing up. Guided by their wise grandfather, Mori Shintaro, who was once a champion in a prestigious martial arts tournament fifty years ago, they strive to master their ninja skills. Their excitement is only slightly dampened by Tum-Tum's fixation on the sumo wrestlers, whose legendary gluttony parallels his own relentless appetite.
As they prepare for an important trip to Japan where their grandfather hopes to retrieve a treasured dagger to present to the new champion, tension begins to build. Mori's yearning to revive past glories clashes with modern realities, particularly highlighted by a dramatic theft at a local museum. The elusive thief, ultimately revealed to be Koga, the boy who once defeated Mori, escapes with a priceless sword, using a hang glider for his getaway.
Back at home, the boys find their baseball game filled with distractions. Rocky finds himself distracted by a captivating young woman, which affects his pitching, while Tum-Tum's incessant craving for snacks leads to frequent game interruptions. Colt's fiery temperament culminates in a brawl, resulting in an unfortunate postponement of their travel plans. With the weight of growing up on their shoulders, the three brothers must figure out how to maintain their sibling bond through the various challenges they face.
After Mori set off for Japan, a group of mischief-makers, led by Koga's clever nephew, Glam, attempts to invade the peaceful safety of his cabin to steal the dagger. Demonstrating their resourcefulness, the boys foil the thieves and brush off the incident as mere vandalism. Unbeknownst to Mori, he accidentally takes Tum-Tum's luggage with him, thanks to a well-meaning yet impulsive friend. Upon arriving in Japan, Mori's taxi is violently side-swiped by Glam and his gang, who make off with his possessions. At the hospital, Mori leads the boys in with the shocking news that they now possess the valuable dagger, which leads them to embark on an unexpected adventure using Mori's credit card and reuniting with him in Japan. They learn from him that the dagger must be delivered to the Grand Master of the tournament. Meanwhile, Koga learns of this development and starts plotting against them.
During the tournament, unexpected drama unfolds when Colt stands in for an injured fighter, only to face a formidable female opponent, Miyo Shikigawa. Although he loses, Miyo helps the brothers in delivering the dagger and offers them shelter with her family. A baseball enthusiast, Miyo impresses with her strength at the plate but admits to struggling with fielding. The brothers propose a trade: they will coach her in baseball in exchange for martial arts lessons. As a romantic spark ignites between Rocky and Miyo, their moment is interrupted by the arrival of Colt and Tum-Tum.
As Koga schemes to con the boys, masquerading as the Grand Master in a bid to reclaim the dagger, their cleverness allows them to outsmart him and escape his clutches. Meanwhile, Ishikawa captures Mori from the hospital, pulling him away from Glam and his gang. Koga then pressures Mori to reveal the legendary Cave of Gold's location, an urban myth thought to hold treasures accessible with the combined power of the sword and dagger. With the well-being of Mori's grandsons at risk, he reluctantly agrees.
Despite prior setbacks, the children devise a bold plan to infiltrate Koga's hideout using DIY hang gliders. Upon arrival at the cave, they uncover the reality of the myth, as gold-laden walls glimmer in front of them. As a fierce showdown between Koga and Mori erupts, the trio intervenes, with Colt displaying his ability to apply Mori's teachings just in time to avert disaster. Yet Koga's greed leads to his own downfall as the cave begins to collapse under the pressure of his actions, prompting a hasty retreat.
On their journey back home, Rocky is met with the pleasant surprise of having an additional day at their disposal, offering a faint hope to join in the championship game. Come game day, each sibling confronts their personal challenges. Facing a two-run deficit in the final innings, an opposing batter connects with one of Rocky's throws, but Miyo shines when she snatches the ball mid-flight. In the final moment of the game, Colt harnesses his focus to hit a dramatic home run, clinching victory for their team.
However, as the game concludes, tension brews between Miyo and a rival player named Darren, who blames her for derailing his home run. She bravely agrees to face him, prompting the boys to prepare for a confrontation. Just as they gear up to defend her, the screen fades to black, leaving the outcome of this climactic moment hanging in suspense.
