The game is divided into two, quite distinct gameplay experiences. On the bottom screen Mario and Luigi explore Bowser's 2D side-scrolling insides as they attempt to rescue Princess Peach from whatever grisly beasties she might encounter. On the top screen, Bowser punches and stomps his way through the 3/4 overhead-view Mushroom Kingdom as he tries to wrestle control of his castle back from the hilarious Fawful. The game switches between the two regularly, with X and Y controlling Bowser, and A and B controlling Mario and Luigi.
The game's at its best when Bowser interacts with Mario and Luigi, and vice versa. For example, in one section Bowser can drink to fill his gut with water, which enables Mario and Luigi to swim up to previously unreachable platforms and solve puzzles that riff on the mechanic. Conversely, in some battles, Bowser can inhale his enemies, which then appear on the bottom screen as foes for Mario and Luigi to stomp on.
Most (perhaps too much) of your time, however, will be spent fighting monsters in bog standard turn-based battles triggered by touching enemies as they loiter about the Mushroom Kingdom and inside Bowser's body. Here, Mario and Luigi, or Bowser, are positioned on the left of the screen, and take it in turns to perform commands you select. Mario and Luigi can jump on their foes, hit them with a hammer, perform special co-op attacks and use items as need be. Bowser can punch, breath fire and use special minion attacks.
Unlike traditional, old school turn-based battles where you input a command then sit back and watch the show, the fights in Bowser's Inside Story require constant attention and timed button presses. Mario and Luigi's basic jump attack, for example, can hit twice if you press the button just as you land on the monster. Bowser's fire breath is a charged attack - press the Y button the millisecond he flashes and it will cause the maximum, "Excellent!", damage.
There are also defensive and even counter-attack moves. Bowser can punch some enemies as they attack, or defend with his shell. Mario and Luigi can jump to avoid attacks. Each enemy has a "tell" which, if you look closely, gives away its attack pattern and the timing required to avoid or counter it.
The problem, however, is that while you're always involved, be it in the battles or mini-games or exploration or in the wonderful cutscenes, you never feel truly engaged. The game's particularly easy, and the RPG elements have no substantial impact on gameplay. Levelling up is inconsequential because the game decides how your statistical increases are allotted. Finding new gear never excites because the boosts they grant aren't required to win. And the special attacks, gradually gained, are pointless because the use of basic attacks is, most of the time, the quickest way to win. The game lacks strategy and depth so markedly, that the RPG elements need not be there at all.
This, coupled with a stringently linear design that wrestles away any semblance of control over proceedings you may have had, forms an experience that lacks substance, even if it is always great fun. You feel like you're gliding through the game on auto-pilot, free to enjoy the wind blowing through your hair without worry of turbulence.
The game's still great, though, in no small part to the wonderful dialogue. Fawful's incorrect syntax in particular is genuinely funny ("Beef? I am lacking in beef. Fawful is beefless", and "A WINNER IS YOU!"), and Bowser, for once more than a demented boss, has his moment in the sun and reveals himself to be a likeable, if somewhat single-minded, anti-hero ("Did somebody page the king of awesome?"). The text localisation team has done such a good job, Bowser's Inside Story will go down as the funniest Mario game of all time.
In truth, this bizarre adventure is tailor made for Nintendo fanboys. It's packed with in-jokes and Nintendo references that'll soar above the Brain Training crowd's head like a Star Powered aeroplane. Its greatest achievement is its pacing: it never loses focus over the course of its 20 hours. It constantly surprises with clever mini-games and stand-out, memorable moments, most of which, it must be said, involve Bowser. But it feels more like a cleverly strung together series of mini-games disguised as a multi-faceted role-playing Mario game, than the multi-faceted role-playing Mario game some might have hoped for. This might be a deliberate concession to accessibility on Nintendo's part, but it doesn't sit well with the tone or the plot. The result: you'll fondly remember exploring Bowser's colon, but you'll feel strangely empty when you emerge on the other side.