High above the sky lies the Midair Stadium, a prestigious venue where the biggest events in the Nintendo multiverse are held. Today, a huge crowd has gathered to watch a match between Mario and Kirby, two experienced fighters with massive followings. Two princesses make a special guest appearance: Princess Peach and Princess Zelda. The match is so popular that even a certain angel high in Skyworld is spectating the match. However, what started out as a friendly battle between two seasoned veterans will soon be the start of a new global threat...
The big fight
You'll first play a battle in which Mario takes on Kirby. You get to choose which character you want to fight as, and both players will get the same characters and be on the same team in two-player mode. The objective is simple, take a stock off your opponent. This is just a standard match against a single opponent, and your higher amount of stocks gives you a big advantage. You should be able to win the match without much effort, and only lose a stock or two on higher levels.
Shadow Bug invasion
After the battle, Meta Knight's ship, the Halberd, will appear and drop of a payload of Shadow Bugs, which form into an army of Primids. Peach and Zelda will join Mario and Kirby to fend off this threat, giving you four characters to choose from. The battle that follows is a free-for-all against tons of enemies. Other than the normal Primid, which only use melee attacks, there's also Boom Primids, who can throw projectiles, Sword Primids, who have longer reach, and Spaaks, clouds that shoot electricity at you. The Spaaks are the biggest threat as their attacks do high knockback and they get more powerful at low life, but you can dispatch them more quickly by attacking the battery on their backs.
Having survived the onslaught of enemies, the group faces the leader behind the attack, a mysterious cloaked figure called the Ancient Minister. Carrying a deadly subspace bomb, he drops his payload on the stadium. Two R.O.B.s activate the device, triggering its three minute countdown to detonation. Mario attempts to stop the timer, but a lone cannonball knocks him out of the stadium. As Kirby watches his former opponent soar into the clouds, Petey Piranha captures and imprisons the two princesses, and it is now Kirby's job to free them.
Boss Battle: Petey Piranha
As the first boss in the game, Petey's attack pattern is quite simple. He will mostly grind one of the cages across the ground, knocking you away if you're caught in its path. He can also perform a high jump and crash back to the ground, dealing a powerful meteor smash if you're hit. The first attack can be evaded by staying in the air, while you can dodge the ground pound by jumping and air dodging. Once Petey has lost some of his health, he will roar and begin the second phase of the battle. The strategy is still the same, except that Petey's attacks are faster and he can do his ground pound move multiple times in a row.
Since you'll only use Kirby if playing through the stage the first time, you easily have enough speed to respond to Petey's attacks, while your multiple jumps let you dodge the cage smashes. As easy as he is, don't get too confident, as getting hit by his attacks on high difficulties can KO you in one hit. Another notable feature about Petey is that he has no life bar of his own, but two life bars for both cages. Attacking one cage reduces its life bar, while attacking Petey directly results in reduced damage being dealt to both. Be sure to attack the cage that contains the character you prefer, as that character will be the one in your roster for much of the Subspace Emissary. If you actually manage to reduce both cages' life bar to zero simultaneously, the character you get is selected randomly.
Having been defeated, Petey falls in a cinematic explosion as both cages break. Kirby manages to rescue one princess, but the other is knocked on to the ground, disorientated. Out of a sudden, Wario appears as a henchman of the Subspace army, armed with a Dark Cannon. He targets the vulnerable princess, turning her into a trophy, then makes a quick escape. Kirby and the other princess tries to pursue him, but with the Subspace Bomb about to detonate, Kirby has no choice but to abandon the chase. In the nick of time, Kirby and the remaining princess flies to safety on Kirby's Warp Star, as the Midair Stadium is engulfed by the dimensional rift created by the Subspace Bomb.
After finishing the level, you will get your first two characters for the mode: Kirby, and the princess that you chose to rescue, which can be either Peach or Zelda. However, with the stadium sucked into subspace, you can no longer replay the level until the end of the story.