From Wikipedia
Story - One day in Feudal Japan, Kid Niki, the most radical of ninjas, is training at his Ninja School. Suddenly, a passing bird is struck down by an arrow and lands at Niki's feet. Attached is a note explaining that Niki's girlfriend, Princess Margo, has been kidnapped by the evil Stone Wizard. With the cry of "Will help you!" Niki bursts through the wall of his school and sets off on his quest to save Margo.
Gameplay - Kid Niki is armed with the Spinning Sword, which according to the game's manual, "has been passed down from generation to generation from the School of Chirin." In addition to the spinning sword, Kid Niki can gain extra offensive power by collecting Bells. The Golden Bell allows him to launch a projectile every time Kid Niki spins his sword, and the Silver Bell creates a spinning force field around him. Both of these power-ups last for a limited amount of time.