A Mouser in the Houser
In Princess Toadstool's Castle, Mario, King Toadstool and Princess Toadstool are in the midst of getting dressed for a formal event, with several Mushroom People attending to them. As King Toadstool begins to tell Mario and Princess Toadstool to be on their best behavior at the party, he gives Mario the Sacred Cylinder of Cheese, a precious item that must be delivered to an ambassador.
Leaving the castle, Mario and Princess Toadstool enter a nearby Warp Pipe, while discussing the damage that Mouser has been doing to the Warp Pipe system. Traveling through a tunnel, Mario and Princess Toadstool continually pass through garbage and poorly placed construction equipment until winding-up lost, with Mario wishing he had his Dirk Drain-Head compass.
Eventually, Mario and Princess Toadstool find themselves atop a castle, with Princess Toadstool complaining that Mario has gotten cheese on her dress. Princess Toadstool, angry with Mario, proceeds to push him off the castle tower, while a figure watches from the shadows. With Mario gone, the figure reveals itself to be what appears to be Mouser; a surprised Princess Toadstool suddenly notices that she's surrounded by Mouser look-alikes.
The mice who surround her proceed to tell Princess Toadstool that Mouser was once their king, until he betrayed them; after saying this, the mice proceed to show Princess Toadstool several carvings on a wall as Mario falls down a Warp Pipe with several Beezos after him and the Sacred Cylinder of Cheese.
Princess Toadstool's mice captors begin to tell her their origin; they were the ones who created the Warp Pipe system and the ones who hid the various treasures in it. The mice proceed to tell Princess Toadstool they were originally ruled by the Mouser family, until the current descendant of the Mousers, Mouser, betrayed the mice and joined King Bowser Koopa, leaving his people to degenerate to the point of stealing vegetables and scraps for food without a leader. As the mice tell Princess Toadstool this, Mario begins to fight his way through a maze of tunnels.
The mice, after explaining everything to Princess Toadstool, begin to follow her every whim, giving her whatever she wants. When Princess Toadstool asks for cheese though, the mice become erratic, saying they have mountains of cheese to the point of almost drowning in it; a mouse goes on to say that they're given so much cheese on a daily basis that its starting to oppress them. After his ranting, the mouse suddenly asks Princess Toadstool if she wants to be their queen, much to her shock.
At first, Princess Toadstool refuses to become the queen of the mice, but when offered a detailed map of the Warp Pipe system, as well as an army of mice to help her defeat Bowser, she begins to consider. Down below where Princess Toadstool is, Mario appears, saying he and the Sacred Cylinder of Cheese are safe; accidentally sending several Sparks away to get ready, when they mistake what Mario says for "Koopa is due?!!"
Hearing Mario, Mouser suddenly appears and begins an attack him with Bob-ombs, believing Mario to be exhausted after all his traveling and battling. Princess Toadstool, seeing Mario's predicament, agrees to become the queen of the mice, if they help her rescue Mario. The mice refuse to help Mario though, saying that, to be their queen, Princess Toadstool has to put them first before anything else. Hearing the decision of the mice, Princess Toadstool refuses to be their queen and leaps off the tower she's on to rescue Mario.
As Mouser begins to throw more Bob-ombs at Mario, Princess Toadstool, grabbing the Sacred Cylinder of Cheese, throws it at Mouser. Mouser, disgusted and horrified by the cheese, promptly flees at the sight of it.
Later on, at the party, as Mario stirs the punch bowl with his plunger and Princess Toadstool haphazardly serves dessert on a cart, King Toadstool proceeds to tell the visiting ambassador that Princess Toadstool managed to defeat Mouser single-handedly, meaning the Warp Pipe trade routes are safe; King Toadstool goes on to say that Mario has lost the Sacred Cylinder of Cheese, though the ambassador responds that he doesn't care that that "stupid cheese log" isn't here.