Revolutionary classic that stands the test of time
SM64 is about Mario going to visit Peach's castle, but he arrives to find that Peach has been kidnapped, and the castle has been taken over by Bowser! So he traverses the castle and the many areas it leads to and sets off to collect enough power stars to reach his final battle with Bowser to rescue Peach. There are 120 stars in all, and it's quite challenging to collect them all.
SM64 was considered revolutionary for several reasons. One, the graphics. They're very nice for the system, but are certainly not as good as, say, the Crash Bandicoot games' visuals. Still, they are very nice looking for the time. Next, the 3-D platformer gameplay. Considered revolutionary for the time, Mario 64 really defined its genre of 3-D platformers. With simple run-and-jump gameplay and loads of other little moves like wall jumps (now a Mario trademark) and ground pounds, there are many moves up Mario's sleeve. The last reason I want to point out is just how big it is. 120 stars to collect was (and is) a lot to put in a game. It'll last a long time if you want it to.
Super Mario 64 is really an unforgettable game. Even if you don't consider it to be revolutionary, you'll undoubtedly consider it a classic. You'll never be able to forget running around the castle, jumping into paintings, and visiting so many worlds in the game. So Super Mario 64 is worth your time and money, and with the Wii Shop Channel, you can get it for just $10. If you have never played this (or the DS version), I recommend you buy it.