This game is one of many ports of an arcade game (there is also a movie of a different version of the game here). In it, an evil force has corrupted all ninjas except for you, and you must defeat the evil force. You have four different weapons, and you can also obtain shadow clones which work similarly to the options from Gradius or the clones from Ninja Gaiden 2.
Sit back and enjoy the fast-paced ninja action and the weapon variety.