As in Super Mario World 2: Yoshi's Island, Baby Mario and the Yoshi clan must rescue Baby Luigi, who was snatched by Bowser's minion, Kamek, who also wanted to kidnap every baby around the world.
In post-credit scene, the seven star children are revealed to be Baby Mario, Baby Luigi, Baby Peach, Baby Donkey Kong, Baby Wario and Baby Bowser as well as Baby Yoshi, who just hatched from an egg.