Rocky, Colt and Tum-Tum are experiencing the pains of growing up prior to a trip to Japan planned with their grandfather Mori Shintaro, who hopes to take them to a martial arts tournament of which he was the victor 50 years ago. Only Tum-Tum seems interested in going, and even then, only out of interest in seeing Rikishis due to how much food they get to eat. He tells the boys he hopes to return a dagger awarded to him at the tournament when he defeated a boy named Koga, so that it may be presented to the incumbent victor. In Japan, a man (later revealed to be Koga) breaks into a museum and steals a sword before escaping via hang glider. Meanwhile, back at Mori's house, a trio of burglars led by Koga's nephew Glam try to break into the house to steal the dagger. The boys manage to drive them off, counting it as an ordinary robbery attempt.