The Underwhere is the main setting for the first half of Chapter 7 in Super Paper Mario.
The Underwhere is where those go after their games have been ended to be judge whether to stay there or in the Overthere. It is ruled over by Queen Jaydes who has the ability to restore the life of others if they can be restored. There is also a fountain that restores HP and the infamous River Twygz where anyone unfortunate to fall in and not come out become Underhands.
Chapter 7-1: Subterranean Vacation
Mario, Peach, and Bowser were all sent to The Underwhere after Dimentio destroys them. Mario wakes up and eventually finds Queen Jaydes who is surprised that Mario isn't actually fully gone. She then agrees to fix the Pure Heart also telling him that someone else has entered The Underwhere not too long ago. Mario searches though the River Tywgz and runs into his younger brother Luigi. Luigi does not remember what happened but agrees to join up with Mario on his quest. The two plumbers find Luvbi who even turned away from them. Mario and Luigi head back to Queen Jaydes who fixed the Pure Heart. Luvbi was infuriated with Mario and Luigi following her, Queen Jaydes tells her to apologize to Mario and Luigi after they find them. Queen Jaydes opens the portal back to the living. Mario and Luigi both leave The Underwhere to place the Pure Heart. The two heroes return back to the Underworld with Tippi and another Pixl. Eventually, Mario and Luigi find Queen Jaydes' daughter Luvbi who reluctantly joins them on their quest to find the Pure Heart. The group then heads out for Underwhere Road.
Chapter 7-2: The Sealed Doors Three
Mario, Tippi, and Luvbi begin their travels to The Overthere, using the Underwhere Road. Before beginning, however, Luvbi asks Tippi if she believes in the idea of a soul mate. Tippi, suddenly becoming flustered, does not answer. This caused Luvbi to count her as undecided, or “unrequited”
Mario begins following Luvbi through Underwhere road, which they much travel up to reach the overthrere. However, upon reaching the door to the next room, Mario is questioned by Dorgu the First. After answering his three questions, Mario is allowed to enter the next room.
They are stopped by three guards, standing sentry next to one of two doors in this room. They said they must lock the door to The Overthere to keep “The Beast” contained. They say the only way they can unlock the door is for the beast to be tamed. Mario accepts the challenge, and enters the room they were guarding.
“The Beast” happens to be Bowser. Still dazed and confused by having his game ended by Dimentio, he is surprised to see Mario. Bowser then questions why Princess Peach is not with Mario. Before letting him answer, Boswer accuses Mario of trying to split them up, and ops to fight Mario instead.
Upon being Boswer defeated, Mario explains the situation. Bowser grows angry, telling Mario that they should have spent this time searching.
They exit, and the guards are surprised Mario had managed to take the beast. Two of the three guards exit to search for more monsters, but the third advises Mario that torches can be lit using fire. However, he warns the torches will only be lit for a limited amount of time.
Mario and co. keep ascending the Underwhere Road, Bowser using his fire to light the torches. As they ascend, they are met by Dorguy the Second, who asks five questions.
Being allowed access once more, the group comes to another locked door. They enter a door covered in hearts, which is home to Hagara. She teleports you to retrieve a book from a certain D-man. Upon doing so, she gives you a key to the door to reach the next room.
The group ascends once more, finding themselves in an outside-like area. Upon reaching the end, Mario and co. are confronted by Dorguy the Third.
Dorguy the Third forces you to fight The Underchomp, a tri-colored, tri-chainchomp. This fight takes place in the style of a typical RPG.
Beyond this final Dorguy is the star block, and the end of the chapter.