Super Mario U is the sequel to Super Mario World and New Super Mario Bros. U. Unlike it's predecessors, this game takes place in another world: Bowser's Doomsday World.
Mario,Luigi, Peach, and the Toads are celebrating the defeat of Bowser and the Koopalings in NSMBU, until a portal appears and Bowser's airship appears out of it. The Koopalings use a metallic claw to kidnap Peach, once they do they disappear in the portal. The gang also stumble upon the portal, end up in an unknown planet and begin chasing after the ship.
Corrupted Plains: The Adventure Begins
They start off in Corrupted Plains where their friend, Yoshi can be ridden. After going through a grassy plain, a cave, and an field with Yoshi they made to the fortress with spikes and gears, where the first Koopaling, Larry Koopa is found. He is quickly defeated and he retreated to his own castle. They continue through a beach, the sky, and a field. Now they made it to Larry's Castle. After safely making it, they made it to Larry again who has a big mech: The Mecha Larry 3000, which could kick footballs, and ram. Mario destroys it defeating the youngest Koopaling, they move on to the next world ruled by Lemmy Koopa, Blazing Desert.
Blazing Desert: The Quest Continues
They continue in Blazing Desert, a world with a hot sun, pyramids, and a giant oasis. They went through a land with ruins, they made it to the second Koopaling, Lemmy Koopa who hides into pipes and throws bombs, he was defeated and using the Koopa Kopter flies to his castle, Mario and friends continue through the hot desert (however making it to the oasis) and makes it to Lemmy's Castle, they made it to a rematch with Lemmy, who is now operating a robotic spider who fires sand balls and sticky web lines. Mario destroyed it sending the small Koopaling flying. Now they made it to the third world conquered by Roy Koopa, Sunshine Island.
Sunshine Island: No Time For A Break
The team had now made it to the peaceful (but deadly) Sunshine Island. The've had made it through the islands and seas, and made it to the tower where the third and brutal Koopaling, Roy Koopa guards. They've took him down after a tough battle. They then continue through a haunted shipwreck, and more islands. Now they made it to Roy's Castle. They have fought Roy who controls a purple submarine which fires homing missiles and is very fast. The Bros. us their own Marine Pop to destroy it. Finding blueprints in the wreckage they've found where they are and how bowser is going to destroy in the Mushroom Kingdom. They now going to the Poison Jungles where Iggy Koopa is waiting.
The team has finally made it to the huge rainforest where the the fourth Koopaling, Iggy used poison to flood the entire jungle. They begin the search to find him. The team explored through a jungle, a toxic cavern, and a huge temple guarded by Boos. The made it to the fortress where Iggy is where he shoots dangerous purple fire at Mario. They defeat him again and he fled to his castle. A giant Chain Chomp kidnaps Luigi, and the Toads. Mario got very angry at chased after the Chomp by chasing him through rapids and rainy forest. He saw the Chomp run into Iggy's Castle. He fought Iggy Koopa again in a Chain Chomp mecha. Mario destroyed the Chomp and Iggy screams in anger and Mario punches his Copter sending him flying. Mario saved his friends and continued through the Ice Cream Glacier where Wendy is ruling.
Ice Cream Glacier
The team has found a mountainside with blue hills and giant ice cream cones. Its called Ice Cream Glacier where Bowser built this land for his bratty daughter, Wendy O Koopa. They went through snowy fields, caves and climb a giant ice cream cone. They finally found a remote ice fortress. Wendy was waiting on the top of the tower. She was defeated and flew away crying to her castle. Mario and the guys have found a ghost house where Kamek is making Boos to stop the Bros. Kamen was defeated, furious he flew away. They have finally found Wendy's Castle. Wendy is in a ice dragon who wants to freeze the player. They ice dragon was destroyed, Wendy flew away throwing a tantrum. Bowser contacted the Bros. saying that he will destroy the Mushroom Kingdom in 6 hours. Mario destroyed Bowser's Telexed and went to the Command Canyon where Morton is guarding.