In which you play as Toad, the badass warrior/mage of a powerful kingdom. At the start of the game, the heiress to the kingdom's power is abducted after a long and violent conflict with various demonic forces (for her soul or whatever), and your character goes on a stealth mission to kill her captors and free her. However, upon accomplishing this, fairly brief part of the story, it is revealed that the heir to the Mushroom Throne is held captive by a complex enchantment that can only be broken through by one specific 'hero'- A delusional plumber who had fallen in love with the heiress, but is too busy hallucinating in the aftermath of the abduction and war to save anything. Another catch is that the rescuer must not have had any significant help in the rescue mission. Toad finds that the loophole in this, however, is that Mario may have support, but he must remain entirely under the impression that he is completing the mission himself. From there the main aim of the game is to follow Mario's path from the dangerous side paths, giving him vague hints while in disguise in a humiliating costume, destroying the more powerful obstructions in his way and deforming or mutilating the weaker opponents so that an insane plumber can defeat them while not giving yourself away by making it a complete cakewalk. You have to leave strategically placed items in his way so that he'll stumble over them and use them to overcome the 'challenges' he faces, and occasionally relocate or become horrors while guiding him through parts of the journey that you've swept clean. Eventually, during endgame, you have to ransack the demonic castle and cripple their (boss) leader precisely to the point where it's in Mario's ability to single-handedly take down an entire fortress (or destroy their leader and leave a weakened sub-boss where he was). Then, when the heiress is released from the magic and escorted to your native land, you can finally kill him and enjoy wreaking destruction post-game without some random restricting you. So basically it's a game in a terrifying and dangerous world where you have to carve out a path for someone else's adventure from the wreckage as you go, and maintain that path, and keep that adventurer shielded from intrusions by reality.
...Nevermind, I can dream.