Tip: Layer 3 backgrounds and tides disappear when you hit a message box, so plan your level around that. For example, you can add a sublevel without layer 3 at the start/midpoint, and include your message there.
Halloween is in the Mushroom Kingdom and Mario goes out trick-or-treating with Luigi. After a while, Mario got bored and he went home to pass out candies. When he came back out, nobody was coming around so Mario went to go check to see what was happening.
Update (02/02/2020): Fixed music issue in accurate emulators.