Mario Galaxy 2 began life as an expansion to the first Mario Galaxy and, when the developers found themselves overflowing with ideas and levels, it was expanded to become a very rare item: a direct sequel to a Nintendo game. A storyline doesn't seem to figure much into the action—Princess Peach bakes something, then gets kidnapped, and Mario has to find her with the help of a star that allows him to spin during his jumps. This makes perfect sense, right?
Mario soon gains control of a starship shaped like his head, and this becomes his home base. You'll be able to pick up friends to journey with you, and you'll gain the use of a bank for your collected star bits; in between missions, you'll be able to gain a few extra lives here and there. You'll need them: this is a game that doesn't mind throwing you into some incredibly tough situations.