Quite possibly the greatest video game of all time
The story is about when Bowser kidnaps Princess Peach before Mario and his trusty sidekick Yoshi come to save her while exploring the cosmos.
Play as Mario as you can explore six main worlds and 42 main galaxies. When you are first in Sky Station, you can explore it and defeat the pee-wee piranha thing and get the first Power Star. Just the way you can discover the next galaxy which is Yoshi Star.
Yoshi Star is where you saddle up with Yoshi. You can also explore this galaxy by eating foes with Yoshi to collect lots of star bits.
The final galaxy which is Bowser's Galaxy Generator is very hard. You must explore the final galaxy and defeat the last boss which is Bowser to get the last Grand Star. After you defeated Bowser, Princess Peach will be rescued by Mario forever. And Rosalina will appear. I know Rosalina is my favorite character.
Super Mario Galaxy 2 is truly one of the greatest games of all time. It's a must-play for gamers. I gave this game 10 out of 10.
EDIT: Super Mario Galaxy 2 might be even better than the original Super Mario Galaxy, but because of Rosalina, the original is much better than the sequel.