So I've had this Mario fangame idea in my head for ages, chronicling an aerial invasion of the whole entire Mushroom World, not just the Mushroom Kingdom or Dinosaur Land or Jewelry Land. The WHOLE thing. This is World 1, Mushroom City, based on and containing the track from Mario Kart Double Dash.
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The story begins with Mario, Luigi, Peach and Toad looking into the future in a fortune-teller's crystal ball... It's Bowser's 50th birrthday. Bowser Jr. and the Koopalings want to give King Dad a birthday present to top all birthday presents, but in order to do that, Mario and Luigi have to be out of the way. Bowser Jr. dons his Shadow Mario disguise and kidnaps Diddy Kong with it. DK thinks it's the real Mario and kidnaps Peach, demanding a trade. Toadsworth goes and interrupts the real Mario and Luigi's golf game, and sends them into the city, where DK is holding the princess.

Level 1, the construction site
This level has four parts, that are rough 3-D interpretations of the Donkey Kong arcade game. Whenever Mario or Luigi get to the top or end of one part, DK takes Peach to the end of the next part. After reaching the end of the fourth part, and taking the floor out from under DK, the level ends, and you're treated to a cutscene of Mario, Luigi, and Peach celebrating victory over lunch, in a little café. But then...

Level 2, main street
...there's commotion outside. Monkeys are tieing up Toads and holding them in trucks. It's up to Mario, Luigi, or Peach to save them. Trace a route along both sides of the street, watching out for traffic, as you find three trucks with tied-up Toads in them, including Toadsworth, Toadette, and "the" Toad, who then becomes playable. But the third truck is a trap! The monkeys grab you and Toad and throw you in the sewers, parking their truck over the manhole cover.

Level 3, the sewers
Toad, and whoever you chose for Level 2, are the playable choices for Level 3. In the sewers, bonk platforms from below to defeat enemies up above, and make your way out of the sewers before the, erm... "tide" comes in >_>
Right before the exit, (and after finding a Yoshi egg), you see Shadow Mario blocking your way, and he has Diddy! He summons a giant mechanical Mario, who has three hitpoints, plus fireballs, a hammer, and a jump attack. Defeat him to finish the level. The egg opens to reveal our good ol' green Yoshi friend hiding inside. He becomes playable too.

Level 4, the Mario Toy Company building
DK saw Shadow Mario run inside with Diddy. But when he followed Shadow Mario inside, he gets ambushed and locked in a cage on the top floor. It's up to Mario, Luigi, Peach, Toad, and Yoshi to set him free, even after all the trouble he caused. Cos they're good like that. Find the key and bring it to the lock to unlock the door.

Level 5, Portobello Park
Someone on the street saw Shadow Mario take Diddy into the fun house in Portobello Park, the fairground where the rides cost tickets that you win in carnival games. The Koopa Troopa working the fun house says admission is 3 tickets, but when he hears the news and is about to let you in for free, one of Bowser Jr.'s loyaler minions bumps the Koopa Troopa out of the way and cruelly increases admission to SEVEN tickets. The Koopa Troopa who got bumped out of the way introduces himself as Karter, and helps you get those tickets by winning them in carnival games. He becomes playable too.

Level 6, the fun house
In this level, you're Donkey Kong. Make your way through a mirror maze into one of three pipes. One takes you to a ball pit, another to a cargo net, and a third to some moving floors. Oh, did I mention all of Wirld 1's levels have time limits? Well... they do. Make it to the end and face off against Bowser Jr. to rescue Diddy Kong. And you've cleared World 1! The other seven worlds are longer, don't worry.

Bonus Level, the museum
The museum has rooms to showcase every enemy that appears in the game. It starts out empty, but if Yoshi turns an enemy into an egg and throws that egg at the coresponding exibit, it'll be put on display and you'll be given credit. YAY! This "level" has no time limit and no goal. Enjoy.