3 Ninjas Kick Back

In this poignant coming-of-age tale, three sibling ninjas - Rocky, a 13-year-old novice; Colt, an impulsive 12-year-old; and Tum-Tum, a precocious 7-year-old - grapple with the trials of adolescence. Under the watchful eye of their wise and experienced grandfather, Mori Shintaro (Mori Tanaka in previous installments), they hone their ninja skills and prepare for the ultimate test: a martial arts tournament that holds special significance for Mori, who was once its champion fifty years prior. The brothers' excitement is tempered only by Tum-Tum's singular focus on the sumo wrestlers, whose prodigious appetites are a siren call to his own insatiable hunger.

As they prepare to embark on a journey to Japan, where their grandfather hopes to reclaim a cherished dagger and present it to the new champion, tensions simmer beneath the surface. Mori's desire to relive past glories is tempered by the harsh realities of the modern world, as evidenced by the break-in at a museum, where a mysterious thief - later revealed to be none other than Koga, the same boy who defeated Mori all those years ago - makes off with a priceless sword before vanishing into thin air via hang glider.

Meanwhile, back in their hometown, the boys' baseball game is beset by distractions. Rocky's romantic gaze drifts off toward a captivating young woman, causing his pitches to go awry; Tum-Tum's insatiable hunger for snacks disrupts the flow of the game, necessitating constant timeouts; and Colt's explosive temper gets the better of him, sparking a brawl that ultimately forces the umpire to call the game until next week, effectively dashing their plans to travel. As the stakes rise and alliances are tested, these three sibling ninjas must navigate the challenges of growing up while staying true to their bond as brothers.

As Mori departs for Japan, a trio of ne'er-do-wells, led by Koga's cunning nephew Glam, attempts to breach the peaceful confines of his cabin in search of the coveted dagger. However, the resourceful boys successfully repel the would-be thieves and dismiss the incident as a routine burglary. Unbeknownst to Mori, he departs with Tum-Tum's luggage by mistake, courtesy of Sam's thoughtless generosity. Upon arrival in Japan, Mori's taxi is brutally rear-ended by Glam and his cohorts, who pilfer his belongings. Later, at the hospital, Mori confides in the boys about the mix-up, revealing that they are now in possession of the precious dagger. The boys seize the opportunity to embark on a trip with Mori's credit card, reuniting with him in Japan. Under his guidance, they are instructed to deliver the dagger to the Grand Master of the prestigious tournament. Glam and his cohorts, however, cleverly record their conversation and transmit it to Koga, who exacts revenge upon them for their failure to retrieve the valuable artifact. Meanwhile, at the tournament, Colt unexpectedly fills in for a fallen competitor and suffers defeat at the hands of an enigmatic female opponent, the talented Miyo Shikigawa. As fate would have it, she kindly assists the boys in delivering the dagger to the Grand Master, offering them shelter with her mother and herself. A keen enthusiast of baseball, Miyo excels as a power hitter but struggles with catching. The boys propose a mutually beneficial arrangement: they will train her in baseball if she agrees to instruct them in martial arts. As Rocky develops a romantic interest in Miyo, the two nearly share a tender moment before being interrupted by Colt and Tum-Tum's arrival.

As Koga endeavours to ensnare the trio with deceit, posing as the Grand Master and attempting to reclaim the dagger, his plan is swiftly foiled by the resourceful boys and Miyo, who outsmart him and make their escape. Meanwhile, Ishikawa seizes Mori from the hospital, whisking him away from Glam and the others. Koga subsequently coerces Mori into revealing the location of the fabled Cave of Gold, an urban legend believed to be accessible through the sword and dagger's combined power. With Mori's grandsons' safety at stake, he reluctantly agrees to aid Koga.

Undeterred by their earlier escape, the children concoct a daring plan, which allows them to infiltrate Koga's stronghold on makeshift hang gliders. Arriving at the cave shortly after the adults, they soon discover that the myth is rooted in truth - walls and monuments of gold lay within. As Koga and Mori engage in a fierce battle, the trio drops in, only for Koga to pull out a gun. Colt seizes the opportunity to use Mori's teachings on focus to deflectively send a ball bearing into the weapon's muzzle, causing it to malfunction and trigger a cave-in. The group hastily flees the crumbling site, with Koga ultimately acknowledging the price of his greed and departing without harm.

As they make their way back home, Rocky realizes that they have inadvertently gained an extra day, leaving them with a glimmer of hope to still make it in time for the championship game. Come game day, the boys confront their personal flaws head-on. Trailing by two runs as the 9th inning approaches, an opposing batter notches a hit off Rocky's pitch, only for Miyo to intervene and snatch the ball from mid-air. In the bottom of the inning, Colt refocuses his energies and smashes a home run, securing the win for his team.

As the game concludes, members of the opposing team converge on their opponents, with Darren, their leader, singling out Miyo as the culprit responsible for ruining his home run. She agrees to face him, sparking tension as the boys prepare to defend her. The screen fades to black just as the fight is about to erupt, leaving the outcome uncertain.