Adventure Mode: "The Dreamy Knights"

Prologue
After Shocker Leader III defeated by the KyuuRangers and Kamen Rider Ex-Aid in the Chou Shocker Taisen bonus stage. His body was in Subspace where Smash Bros. Heroes defeated the Subspace army. But some how the remains of the Primids have been resurrected by Tabuu's wish. So then Shocker Leader III meets Ultron Sigma and tells him he will leader of the Nightmare Army. And this was beginning of the Dreamy Knights facing the Nightmare Army.

Chapter 1: New Beginnings
In the Mushroom Kingdom, Mario, Sonic and their friends visiting Peach's Castle then suddenly... Bowser, Bowser Jr. and the Kooplings showed up and he wants marry Princess Peach again. (In Super Mario Odyssey, she refuses to marry Mario or Bowser, much to their dismay) So Mario and Bowser's battle was interrupted by The Nightmare Army's arrival.