Three young boys, Rocky, Colt and Tum Tum together with their neighbor girl, computer whiz Amanda are visiting Mega Mountain amusement park when it is invaded by an army of ninjas led by evil Medusa, who wants to take over the park and hold the owners for ransom. Kids and retired TV star Dave Dragon, who made his farewell appearance at the park at the time the ninjas appeared, have to break Medusa's vicious plans.

During a championship baseball match, the three brothers hear that their grandfather in Japan is in trouble, and head out to help him, conceding the match. When they arrive in Japan, they must use all their powers to defend him against his ancient enemy, who has returned to exact revenge.

After her partner mysteriously disappears, Detective Kelly Jones is lead to a nursery that seems to hold many secrets.