Princess Peach is kidnapped by Bowser and Mario sets off to save her. During the battle, a giant sword crashes into Koopa's Keep. Mario, Bowser and Peach are scattered across the land, and when Mario tries to return, he finds out that an evil invading army of metal creatures are trying to take over the Mushroom Kingdom.
Mario sets off to find the princess and along the way he is joined by Mallow, a cloud who thinks he's a frog; and Geno, a representative of the Star Road in possession of a doll's body. Together they vow to rebuild the Star Road and stop these new monsters at all costs. Later on in the game, Bowser (who wants his keep back at any cost) will join you, and even Princess Peach can be found later in the game.
You fight enemies like in most RPGs - by choosing to attack, special attack, defend, or use an item from a menu. As you defeat enemies you get experience points that let you level up, as well as Coins and other items. The world is huge and contains both old and new characters and enemies. There are also many minigames to be found, providing a diversion from the expanding storyline.
Guerilla & Chained Kong
The enemies Guerilla (JP name: Donkey Yung) in the Forest Maze, and Chained Kong (JP name: Barrel Kong) in Barrel Volcano are based on Donkey Kong, specifically the Donkey Kong Country CGI model.
When you use Mallow's Psychopath ability on a Guerilla, it says "Don't confuse me for someone else!"
In Bowser's Keep (second time), a Guerilla throws barrels at Mario down a series of platforms, in reference to the original Donkey Kong.
Submitted by TV Eye and danipepino.
Culex
Monstro Town has a locked door that can only be opened with a special item known as the Shiny Stone. Behind the door is the hardest battle in the game with a demon known as Culex and four elemental crystals.
Culex is based on similar boss battles in the Final Fantasy series of RPGs, developed by Square (developers of this game). The three songs that play during your encounter with Culex are also taken from the Final Fantasy series.
Contrary to popular opinion, Culex is not from an actual Final Fantasy game. He has been designed to look like boss characters from the series, but it does not appear in any game other than this one.