Mortal Kombat 9 is sort of a reboot to the Mortal Kombat series. It takes place after Armageddon, but goes back in time to when the earlier games in the series happened. After Armageddon, Raiden and Shao Kahn are the only survivors. Right before Shao Kahn kills Raiden, he sends a message to his past self to prevent the armageddon from ever happening. The game is actually called just Mortal Kombat, but to avoid confusion, it's often referred to as Mortal Kombat 9, as it's ninth game in the main series.
The gameplay brings great ideas and it works. It is very nice how you can pull off combos on opponents without having to touch them on the screen like you had to on Mortal Kombat 3 and 4. The combat is also more fast paced, compared to Deadly Alliance, Deception and Armageddon. There is no run button, but it is not very need and I'm actually glad it wasn't included, as online players could just abuse it as much as possible. The fatalities are back as actually fatalities, unlike the Kreate-a-Fatality system in Armageddon and the watered down fatalities from Mortal Kombat vs. DC Universe. There's also the addition of an X-Ray attack, which is a powerful attack that shows a close-up of the opponents' bones being shattered as it's pulled off. It can be used at anytime during a match, as long as all three super bars are full. Most of them can be blocked though. A more powerful version of a special move can also be pulled off using 1 super bar. Overall, these are great additions and should stay around.
The roster could have used more characters from the 3D games, but seeing how as this is a reboot and goes back to before Deadly Alliance, I can see why this did this. The only characters from the 3D games in this are Quan Chi (unlockable) and Kenshi (downloadable content). New to the series are Cyber Sub-Zero (unlockable) and Skarlet (DLC), in addition to the guest characters Kratos from the God of War series (exclusive to the PS3 and Vita versions) and Freddy Krueger (DLC). Almost everyone from the first three games are playable here. The bosses are Goro, Kintaro and Shao Kahn. Goro and Kintaro are fought interchangably in the penultimate battle, whereas Shao Kahn is always fought in the final battle. They are not playable, which is a shame. They could have made them playable and just make the playable versions less powerful than the boss versions of them. When they are fought, sometimes attacks won't phase them, which is bit too cheap, and their X-Ray attacks (which are unblockable) deals massive damage, whereas yours doesn't do much damage to them. But still, they aren't too bad, compared to some of the past games.
For modes, you have Arcade, Story, Challenge Tower and Online. Arcade is the traditional one-player mode. It can also be played using the Tag Battle system, a feature new to the series. Story is the mode that tells the story of the game, and you fight other characters as move along in the story, sometimes with different twists. Challenger Tower offers 200 challenges, ranging from traditional fights, fights with different twists to them them, to various mini-games. Online is where you fight others from around the World and gain points with each fight you win. With all this, the game offers tons of replay value.
Overall, Mortal Kombat 9 is a game that takes the best elements from the old 2D games and the 3D games, as well as incorporating new ones, resulting in the best Mortal Kombat game to date.