Ninja is very brief text adventure by Paul Panks, who was apparently a rather notorious creator of interactive fiction. The game appears to have been programmed from scratch in BASIC, which probably explains why there are some issues.
The writing is very minimal. The objective of Ninja is not explained to the player in the game; but the game consists of only four simple rooms (that I could find), so it wasn't hard for me to figure out what was going on. It seems an idol must be taken from a rival clan's shrine and placed in your own shrine. The game features one simple puzzle (besides figuring out the main objective). It also seems that there is some janky random combat, as I was killed instantly by another ninja on my first attempt to take the idol (I never encountered an enemy again on subsequent playthroughs). There are a couple of glaring errors in the design (some text appears in a room description when it shouldn't, and an important item can't be examined), but the game basically works.
I wouldn't necessarily recommend playing Ninja unless, like me, you find yourself wondering "Who was Paul Panks, and why were his games so infamous?" I'm not sure Ninja answered my questions, but I have at least been given a small introduction to the works of Panks.