Every hundred years a comet passes over the Mushroom Kingdom and rains down stars and stardust and as Princess Peach and the citizens of the kingdom are celebrating the centennial event, Bowser and his legions attack by launching asteroids at the Mushroom Kingdom and crystalizing the Toads. Knowing that Bowser is after Princess Peach, Mario runs to save her. Things go bad as Bowser summons a massive spaceship which abducts Peach's entire castle.

As Mario hangs on to the castle, Kamek confirmed as a boss in Super Mario Galaxy, hits him with a magical attack sending him far away. The next thing he knows, he finds himself on a mysterious moon high above the Mushroom Kingdom. A new character is introduced in the game, Princess Rosalina. She is a Watcher of the Stars who aids Mario on his quest to find Princess Peach, as well as the Grand Stars.
(Wikipedia)

The ultimate Nintendo hero is taking the ultimate step…out into space. Join Mario as he ushers in a new era of video games, defying gravity across all the planets in the galaxy.

Players run, jump and battle enemies as they explore all the planets in the galaxy. Since this game makes full use of all the features of the Wii Remote, players have to do all kinds of things to succeed: pressing buttons, swinging the Wii Remote and the Nunchuk and even pointing at and dragging things with the pointer.

Since he’s in space, Mario can perform mind-bending jumps unlike anything he’s done before. He’ll also have a wealth of new moves that are all based around tilting, pointing and shaking the Wii Remote.

Cheat Codes

Demo version: Bonus levels
Hold 1 + 2 and press A + B at the demo version main menu to play two hidden stages.

Unlockables

Play as Luigi
Collect all 120 Stars then go back to Rosalina at the Comet Observatory and fight Bowser again. Luigi will be unlocked as the main character after the credits.

Grand Finale Galaxy
Collect all 120 Stars with Luigi as the main character and defeat Bowser. The Grand Finale Galaxy is the Star Festival seen at the start of the game.

Star 121
Collecting all 100 purple coins in the Grand Finale Galaxy.


Here's the game's intro along with some gameplay footage below: