Mario Party 9 is simply meant to be played with other people, sure there is a "story mode" if you'd like to call it that. But basically it's a runthrough of all the different boards in the game and a sweet party points bonus at the end.
What exactly is party points? Party Points are the currency in this game and it is used to unlock different vehicles, constallations (which are entirely useless) and stages, as well as some other goodies like Master difficulty for the COM etc.
The game itself isn't your typical Mario Party and is the first of the series to radically change the gameplay mechanics. First and foremost, there is no more collecting 20 coins and going to Toad, there is no more capsules, there is not even individual competition. Oh no, you are strapped in a vehicle TOGETHER. Each turn, a player rolls the dice and is the "captain" of the vehicle, however many spaces the player rolls, all the players go with him/her.
It's a unique idea and makes the game fresh, it also changes the strategy of the series entirely by making decisions which ultimately will either effect yourself, your entire squad, or another rival player.
There is no online play, but then again that is no surprise. With the Wii U coming out this holiday season, there is plenty of room for a Mario Party 10 with Nintendo Network gameplay.
Until then folks, if you are a fan of the Mario Party series, definitely pick this one up. It freshens the concept and is the best Mario Party game in the series that I've played since Mario Party 2. However, it is strongly advised that you have friends or family to play with, despite the offering of single player gameplay, it quickly can become a grind-fest for party points and ultimately dull.