Ninja Boy 2 follows up the story from Ninja Boy, and now includes RPG elements.
In the previous game, the world has been saved and an intergalactic conference has been called. Alien invaders came in and kidnapped all the dignitaries; declaring themselves to be the rulers of the entire galaxy in the process. Several alien leaders became lieutenants in order to make the claim look legitimate. Ninja warriors Jack and Ryu must help the people of Futureland defeat the alien overlords by creating a spaceship from scratch. Despite their intentions, Ryu and Jack must land at all the planets in order to repair the spaceship.
Source: Wikipedia, "Ninja Boy II", available under the CC-BY-SA License.