Probability 0 starts out with you as an unnamed man exploring an infinitely long cave system. The game never explains why, or why you have throwing stars, or why there are several different kinds of monsters out to kill you, but it really doesn't need it. This is gameplay divested of a story. A game where skill matters more than the ability to pay attention, and that's a good thing given the state of gaming at the moment. That being said, even a little bit more background info would be nice, even if it's just descriptions of the little beasties you fight.

The graphics and sound are good, but have a few flaws. One of the major flaws behind the graphics is the accidental inclusion of artifacts in certain images. Another is the change in style between the game itself and the interface. As the main game is pixelated, the interface being quite smooth makes it a rather jarring transition between playing the game and looking at the number of stars you have or how deep you have gone. The health meter takes up way too much space as well. Finally, there are no sounds or music, so putting on your own music while playing is advised.

The basic gameplay of Probability 0 is to survive as long as possible as you descend through a randomly generated landscape, scoring points as you do so. You can jump, throw throwing stars, and punch, with more abilities unlocking as you gain the experience to use them. The alternative to this is getting four guaranteed abilities at the very start of the game with no possibility of advancement. This makes the early game much easier, but later on much harder. While the game makes you think that the depth which you reach is important, it really isn't, as you can get a great score with a low depth or a low score with a high depth.

Enemies come in a few basic flavors. There are chasers, which chase after you until you kill it or it disappears off of the top of the screen; runners, which flee from you if possible; bouncers, which bounce into the air if you pass above it or damage it (such as through a punch); shooters, which fire projectiles upwards that travel through whatever is in their path; and sitters, which sit in one place and slowly drift downwards, hopping occasionally. There are more types as you progress, but these are the general ones and the creatures you should be most concerned about.

Scoring is accomplished in two ways. The first is by killing monsters, with the monster type corresponding to the amount of points you get. Killing monsters gives you a lot of points and drops little pick-up modules, so it is the best way to score. The second is by destroying glass blocks, which are any block with a see-through center. Destroying these blocks gives you two points apiece. What the game does not tell you is that your points tie into your probability, which is what your health is. The higher your health, the better chance you have to survive. If you are low on health, a great way to inflate your probability is to destroy glass blocks to increase it. It's very slow, however, so chances are you will never get to use it.

Whenever you pick up a drop, it automatically gives you the top selection of your drop list. Drops can have one of several effects. They can give you throwing stars or even experience points, with more experience points being required for higher levels. Every time you level up, you can press space to pause the game and apply your skill points to new attributes you would find useful. As every level gives you a new tier of attributes (to a point), you can customize your character as much as you want. There are many different builds possible, from the shuriken-hurling machine to the tough brawler, and none are really invalid thanks to the game's strong sense of balancing.

The gameplay of Probability 0 suffers from a few big flaws. The first is the lack of a definite health meter. The probability number at the top of the screen acts like this, but it's inconsistent and hard to read unless you are near death. The more health you have, the more random the number gets, which is a little frustrating. There are also some control issues, with massive knockback being a major one. The ability that makes you immune to knockback while on the ground is nice, but it doesn't work in the air, which means if you take any damage at all you will most likely fly off the screen and die. The depth meters can be a possible frustration as well, as they randomly switch around.

Probability 0 is a small game that most can play for a few minutes and get a good experience from. It has minor gameplay issues, some graphics errors, and a lack of sound, but the overall presentation is addicting and enjoyable for such a simplistic game. It's small, the average session lasts a few minutes in any mode, and the general gameplay and graphics mixture has a lasting appeal that you can't really put your finger on. You can download the game from the TIGSource feedback thread.

For another look at freeware games, take a peek at Joystiq's Free Game Club weekly feature!