Nintendo's perennial hero Mario is looking flat these days. That's the conceit in "Paper Mario: the Thousand-Year Door," a role-playing-style adventure starring a two-dimensional Mario in a 2.5-dimensional world.

Why 2.5-D? Because while the backgrounds look 3-D, and Mario can move into the foreground and background of the game's locales, Mario gets to and through places mostly by traveling left and right. Mario's depthless qualities are well implemented in the game play; more on them later.

As to the story: Princess Peach has been captured (in an interlude, she laments putting Mario through the worry again), though not by the Koopa King, Bowser. No, this time she's been nabbed by a weird group of roundish villains calling themselves the X-Nauts. Just before her abduction, Peach sends Mario a map and directions to the town of Rogueport, a sort of wretched-hive-of-scum-and-villainy-lite that's rumored to hide a great treasure. That treasure is locked behind the titular Thousand-Year Door, and the keys to that door are several Crystal Keys hidden throughout the land.

Guess where the map leads? So off Mario goes, quickly making a few friends. One, Professor Frankly, is a wizened old Goomba who knows a lot about Rogueport's history. The other is Goombella, a young student of the professor's who teams up with Mario to look for the treasure.

Mario will meet several more characters who will join his party, and all of them contribute in battle and help him proceed. Goombella, for example, can scope out enemies' weaknesses and tell Mario more about who he's talking to and where he is, and Koops, the put-upon Koopa, can bash enemies with his shell as well as trip switches and grab items Mario can't reach.

Battles are carried out on a literal stage, watched by an audience of characters from the game. If they like what they see, they'll reward Mario with enough approval for him to pull off super moves. Occasionally, some ill-intentioned foes will work their way into the crowd and cause mischief if left unchecked.

The battles play out as turn-based exercises in timed button pressing. Mario and his current partner enter from stage left, and their foes wait at stage right. Mario's side goes first; each character can make several regular attacks and special moves, most of which can be strengthened by pressing a button at the right time. When the enemies attack, Mario and friends can defend and even counterattack by the same principle. It's a simple and effective system that keeps the player involved in the combat in a way that many turn-based games don't. Like those other games, however, the battles do get repetitive eventually.

But there's lots to do besides fight, and that's where Mario's two-dimensionality comes into play. The concept is ingenious and expertly integrated into the game play.

While traversing the game's dungeons, Mario finds many places blocked off by bars too high to reach or too low to walk under. To solve these and other quandaries, he learns to form himself into a paper airplane or boat, roll himself into a tube, turn himself sideways and do other folding feats. These abilities let Mario explore new areas as he earns them, though he'll often have to backtrack through places he's been before to reach the new ones, which can get annoying.

The visual style is terrific and consistent. All the characters are paper-thin, like Mario, and they're all expertly drawn and animated. Even the biggest bosses look like they're made up of several sheets of paper, like a paper model kit. Even the seemingly solid backgrounds are revealed not to be so. When Mario enters a building, the walls fall away and fold up like a diorama, only to pop up again when he leaves; and when he presses a switch in a dungeon, parts of the area tear away or are otherwise removed to show the new stuff beneath.

The game's dialogue is clever and self-aware, filled with little comments and inside jokes about other Nintendo titles and video games in general.

Mario's brother Luigi, forever left behind, even pops up now and again to regale his older brother with tales of his own adventures in the Waffle Kingdom.

Mario may lack depth in "Paper Mario," but the game sure doesn't.

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Paper Mario: The Thousand-Year Door

4 stars

SYSTEM: Nintendo GameCube
PUBLISHER: Nintendo
PRICE: $49.99
AGE RATING: Everyone