The game marks Lone Planet's big return to the Super Mario franchise. It is being made with their 2016Bit engine, the same 16-bit engine that was used to develop Sonic 5 & Chill. SMWNX has been confirmed to have a more complex plot than most Mario games.
Mario, Luigi, and Princesses Peach and Daisy decide to go on a vacation to Dinosaur Land -- just one last time. Once all the Yoshis greet them, they settle into Green Yoshi's house, doubting that the Koopa Troop will interfere...
But they do.
Bowser and the Koopalings have uncovered a mystical object deep beneath the mountains of Dinosaur Land -- the Cosmic Egg, which prevents the ancestors of the Yoshis from rising again and wreaking havoc. Mario, Luigi, Peach, and Daisy head out to stop them.
World 1: Hope Savannah
The four heroes come across the Toad Brigade, and their captain explains their current situation: there is no way to leave Dinosaur Land, because the Koopa Troop has formed a blockade around the island. As they go through Hope Savannah, they find a few clues as to what Bowser is up to.
Later on, Mario and his allies come across a fortress, where Larry Koopa fights them. After his defeat, he flees and accidentally leaves behind a strange tracking device. Peach states that this must be what the Koopalings are using to look for the Cosmic Egg.
After a few more levels, the heroes enter a secret base set up by Bowser. They enter the command room, and Captain Toad asks Bowser why he wants to bring back the Yoshi Ancestors. He refuses to answer, saying that he technically already has. He sends out the Miyamotosaur to attack them, but it is easily defeated.
World 2: Dunes of Persistence
Mario and the team enter the Dunes of Persistence, where they meet up with Birdo. She guides them to the caverns, where they uncover a strange essence. Daisy believes that it may be the pure energy that leaks out of the Yoshi Ancestors when they are weak. As they go on, they fight Roy Koopa in a fortress, revealing another tracking device. Luigi begins to wonder why they would need multiple devices, until Mario checks Larry's. It shows the color of the Cosmic Egg, and Roy's describes what it feels like via touchpad. Captain Toad says that there are five, and they need to steal them all from the Koopalings. As such, they fight the Kondosaur in the desert temple.
World 3: Taiga of Glee
The heroes meet the Star Spirits, who tell Mario and Peach that the Yoshi Ancestors are growing stronger with every passing hour. They go with them into Wendy's fortress, where she has locked Toadette. After beating Wendy and taking her device (which reveals which side of Dinosaur Land the Cosmic Egg is located -- the far north), Toadette explains the mythology of the Ancestors:
They were once peaceful creatures known as Katoshis, and were friends with everyone... except the Koopas, who tried to use their spectral essence for their own gain. However, one Koopa named Morton Sr. accidentally gave them too much energy, driving them mad and causing them to go on a rampage. At last, the people of the Mushroom World had to call upon their goddess -- Rosalina, Maiden of the Stars -- to stop the Katoshis. So, she sealed them into the depths of the earth, and created the modern Yoshis to ensure that they would not escape.
Peach declares that they need to find Rosalina, because only she can figure out what to do. The team ventures into the ice palace and fights the Inafunisaur.
World 4: Jungle of Courage
Donkey Kong greets Mario and his friends in the Jungle of Courage, and they go deep within the rainforest to Lemmy and Iggy's fortress, stealing their tracking device after defeating them. It shows the exact coordinates of the Cosmic Egg: 00, 00, 99. Realizing that the Egg is close to the mantle of Mushroom World, the heroes head to the jungle temple and battle the Reggisaur, revealing a stairway up to the mountains.
World 5: Mount Honesty
As the team meets Professor E. Gadd, who has opened a hole to the center of Dinosaur Land, they head for Morton Jr.'s fortress and take his journal. They read the latest entry, which explains Morton Jr.'s belief that someone is taking advantage of the situation regarding the Cosmic Egg. Determined to find out who it is, they beat Iwatasaur at the mountain peak and head underground.
World 6: Loyalty Caverns
Mario's team searches for Bowser and the mysterious villain in the caverns of the land. After beating Ludwig, they find a centerpiece for all five tracking devices, thus linking them together and constructing a strange weapon.
They finally find Bowser deep inside the cave, and he states that he had released the Katoshis so he could destroy the Mushroom Kingdom and build his own dominion over its remains. They fight him, and Mario is about to knock him into the lava until a strange female Koopa punches him away. Bowser is then seen locked up in a large crate.
Lavora Koopa reveals herself, and snatches the odd weapon formed using the tracking device. She explains that this was all part of her master plan -- to absorb the energy from the Cosmic Egg and become powerful enough to become the Queen of Mushroom World. Lavora flicks a switch on Ludwig's centerpiece, and a sharp needle is ejected into the Egg. She places her hand into the socket and grasps its energy, teleporting away with a dark, villainous laugh. Peach exclaims that she must have went to the Mushroom Castle, where she has been scheming this entire time...
World 7: Lavora's Empire
Mario, Luigi, Peach, Daisy, and their allies reach the Mushroom Kingdom after realizing that Lavora created a diversion for both sides so she could achieve her own goals. They fight a brainwashed Boom Boom and a squadron of Reznors before finally reaching Peach's Castle, where Lavora is waiting for them. The final battle begins normally, but the energy she has absorbed soon becomes too much, mutating her into a beast much like a Katoshi.
Fortunately, Rosalina drops in and gives the four heroes cosmic energy, evolving them into their new Omega Forms. Omega Mario and his friends have a long, hard fight with Lavorasaur, until at last she is defeated. Lavora shrinks back down to normal size, and Bowser breaks out of his crate and arrests her, saying that he will never trust her again.
Rosalina restores the Cosmic Egg's energy, and all of the Katoshis pleasantly return to their deep slumber. Bowser apologizes for his misunderstanding, and asks for one thing in return...
the Mushroom Kingdom.
After Peach disagrees, Bowser kidnaps her and Daisy, and the Mario Bros. chase him down, starting another one of their old-fashioned adventures...