So the story is:

(There's a lot of blank stuff cuz I just created this) Someone made a mario maker level, put a plant in a question block, and forgot about it. Fast forward,

Mario grabs a switch on a countertop through the TV,

Makes it a level,

And you have to play it.

The piranha plant starved and didn't have sun, space or water, so it resembles a dead plant, but it can't die because it's trapped in the block.

The start of the song (phase 1) is it banging on the walls because it hears you.

Phase 2 is where you set him free, this could have a dodge mechanic where it trys to bite you to encourage you to kill him. Then the end, you could throw a fireball at it and kill it. I'll explain more if you ask but ima bout to come off my phone so bye :)