The....... *drumroll*

You get to play as Mario, Luigi or Toad
Story:
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You start in your house and a Toad tells you to hurry to Peach Castle.
On your way, there are dead skulls on the path. Peach then tells you that there is a virus that infects the brain, quickly draining away its power, making the body unable to move, blink, breathe, and then the deceased brain absorbs the dead body which leaves a head skull behind.
The player is sent to join an investigation team in World 2, telling you that the Frost Ruins world has a temple with a Magic Leaf that can fully heal and cure people or objects and it can also give you incredible power or resources. To reveal the entrance to the temple, you need all the Power Stars.
(The next part of the story has spoilers, read at your own risk)
-Some levels are harder and some are easier than the others but they all averagely have the same difficulty.
-Every world has a Power Star, either in a Bowser castle or in some ruins. Some will be on the normal path but some will have to be found with a secret exit.
-Before you ask, this doesn't play like those kind of episode where you have a hub and every single room has 10 minutes long levels, no no no, this plays like a normal Mario game where you complete a bunch of moderately short and fair levels then move on to the next world.
Screenshots (Levels in order):
Credits for graphics: