In the world of Smash, the Midair Stadium is just that, a floating stadium where trophies fight. You start this stage by picking either Mario or Kirby and then fighting the other. After Mario or Kirby wins(whoever the user was controlling, dark red clouds fill the sky and the Battleship Halberd flies in. It then releases a large amount of Shadow Bugs which form into Primids. Seeing this Peach and Zelda jump down from the grand stands to help. After beating the army (which oddly included Spaak), a cannonball flies out of nowhere and hits Mario sending him into the clouds. An unseen force then captures Peach and Zelda, leaving Kirby to fight alone. Petey Piranha comes out of the mist holding large birdcages with Peach and Zelda in them. Kirby fights Petey and is soon victorious. Petey catches on fire and falls, presumably dead, to the ground. Kirby helps up Peach or Zelda (depending on whose cage he broke during the fight) and Wario shoots the other with his Dark Cannon. As Wario escapes with his captured princess, the Ancient Minister appears and drops a Subspace Bomb. Kirby and Peach/Zelda then escape on a warp star as the stadium explodes into the purple bubble of subspace.
At the end of the game the Midair stadium is restored.