I just got this game, as it was released in Australia today, and I have to say its an excellent game. The only thing I would have liked to of seen is better graphics, but it does run off the same engine as Superstar Saga for GBA, which is a game I also Thouroughly Enjoyed.
Anyway, the story takes part in Present and Past Mushroom Kingdom, where Professor E. Gadd has developed a Time machine. Princess Peach uses it to go back in time to see her Kingdom before her Rule. But, as the time machine comes back, looking destroyed, strange creatures emerge from it, and the Princess is no where to be found. The Mario Brothers must use time portals opening all over Mushroom Kingdom to Find and save the Princess.
Not very difficult, not that I have beated it yet, but from what I can see. Still, an excellent game.