It's time to go after the Bowser Jrs. Go south and take the elevator back down. The Bowser Jrs. are here. Catch both of them without being spotted by minions. You can't be spotted once. Study the patterns of the Mechakoopas. If you want to go off to the sides, go to the right, as there is a spring platform there. Approach Bowser Jr. to catch him. Go east to find Paper Bowser Jr. You'll find more Mechakoopas and blue platforms moving over the lava. Once across the lava, you'll have to deal with Fly Guys. Dash to get past the long line of them. There are Bowser statues that will move in front of Mechakoopas, so be careful. Once past them, jump onto the blue platforms, then grab Paper Bowser Jr. Luigi retrieves a book from them, but it doesn't look like the book that contains Paper Mario's world. Starlow believes the Bowser Jrs. wouldn't give up the book so easily. They get up and take the elevator, saying the bros and Starlow can't boss them around. They say they're the bosses and Starlow's not even a miniboss.
Go back up the elevator and go north. The bros catch up the Bowser Jrs. They decide to fight this time. They'll each target a bro, then fly to them. Counter with the hammer to knock them out of the clown copters, then repeatedly counter until they get up. They then challenge Paper Mario to see who can fly the farthest. Starlow says to press (Y) when you're about to stop gliding, waiting until the last possible second. Winning will damage the Bowser Jrs. They'll place barrels on the bros' heads and fire at them with their clown copters. Jump to dodge, noting who is who. They'll also send a giant ball onto the battlefield. The bros will use Paper Mario as a trampoline. Press (Y) to bounce objects off Paper Mario. Move around so as to not step on Paper Bowser Jr. They will use this attack even if only Paper Mario is on the field, so leaving the battle to Paper Mario isn't an option here. Move around with the circle pad. The Bowser Jrs. can also heal each other. The Bowser Jrs. will hide in their clown copters if attacked, so some attacks, such as Fire Flower and Bomb Derby won't work. You must attack them from above or use Trio Attacks. When they get really low on health, they will attack multiple times.
After the battle, the Bowser Jrs. and the real book are lying on the ground and the door to the princesses opens. The Marios get a kiss from their respective princess. Luigi wants a kiss, but is ignored. Bowser Jr. wakes up, calling to his paper doppelganger. He flashes back to when he met Paper Bowser Jr. and thinks about him being locked away forever in the book. The Bowsers appear, angry at their sons for letting the princesses wander freely. They've made modifications to the castle. Starlow wants to grab the book, but it's too dangerous. Bowser roars at them for hurting Bowser Jr. He tells Kamek to fire the boosters. The castle starts shaking. Luigi looks around, but doesn't grab the book. The emblem on Bowser's Castle turns from green to red. The ore from Twinsy Tropics Dungeon is being used as fuel to lift the castle in the air. Multiple cannons appear on the castle. Bowser has created another Neo Bowser Castle. Bowser Jr. mentions the book. The Bowsers ignore them. Starlow suggests leaving the book and escaping, since the Bowsers don't seem to care about it.
Outside, it's very windy and the bros, Starlow, and the princesses fly off the castle. Starlow's barely hanging on with her teeth when Kamek attacks her with magic. They manage to dodge all of his attacks. Kamek compliments her teeth. Starlow comments on her dental hygiene, letting go of the castle. They all fly off, this not being the first time a compliment on Starlow's teeth leading to her undoing. Kamek repeats the compliment to himself, making it seem he was somewhat sincere.
On Sunbeam Plains, there's a delivery. He sees everyone fly overhead. They land far away, with Paper Mario carrying them as an airplane. The princesses return to the castle while Starlow wonders what Bowser plans to do with the castle. They need a plan to stop the Bowsers and get the book back. Head towards Peach's Castle and the ground starts shaking. Neo Bowser Castle appears in the sky, along with the Airships. The cannons fire and the cannonballs land near Peach's Castle. Inside, the Toads are panicking in the debris-littered castle. Bowser's taking his revenge on the bros for hurting Bowser Jr. A large cannonball takes off the top turret of Peach's Castle. Bowser credits his flying castle to the abducted, unpaid Toads. He thinks his plan to take over the Mushroom Kingdom is pretty solid. Paper Bowser reminds him this is a joint venture. Kamek reports that the bros should be finished. He also thinks Paper Bowser is useless to them now. Bowser agrees, saying he finds his paper double to be annoying. He concedes Paper Bowser is nearly as strong as him. Kamek has an idea involving the book. Paper Bowser and Paper Kamek have an identical conversation, which means it's likely a race for the book.