In Probability Zero you play a character who has to keep traveling down, doing all he can to stay alive. It's a downward scrolling platform game, and the platforms are procedurally generated, so will never be the same in each game.
Controls are introduced at the start of the game, and are limited to movement, a jump and an attack button. Your character can be damaged by touching enemies or being shot by them. You can also hurt yourself by jumping too far down. Some platforms can be hit to destroy them, and combined with jumping should allow you find a route ever downward.
Hitting and destroying enemies will give you some kind of power-up, so if you survive long enough you can increase your odds of survival. A big number in the center of the screen constantly shows your probability of survival, so the lower it gets the nearer to likely death you are. It's a strange idea that fits with the unusual atmosphere and endless gameplay.
The graphics are basic, and the soundtrack is good, but gets repetitive after a while. The game is still enjoyable, however, and online scoreboards add a competitive element to the retro gameplay.
Probability Zero is a simple yet addictive platform game, that manages to be unique too.