This is the two-player mode. The second player is able to control a Luma to help Mario.
You also have to feed the BIG purple Luma in order that he can give energy to the Starship.
The story:
After the White Luma sacrifices himself to rescue the Universe (end of SMG1) he comes back in the Mushroom Kingdom and fall from the sky. Mario rescue him (World 1-1 is World 1-1 of Super Mario Bros)
Peach invite him to celebrate the first year he has defeated Bowser... Bowser reappears!







