The party enters the Mansion after getting captured by Queen at the end of the Cyber City. Queen takes Noelle using a capture cage in an effort to use her power to create more Dark Fountains, putting Kris, Susie, and Berdly in the rooms of the Mansion's guest hall. Queen does not have a fifth capture cage for Ralsei, so she makes him into one of her butlers instead. Queen instructs the prisoners to stay in their cells while she dominates the world using Noelle, but Kris manages to escape by deploying Lancer from their pocket. Lancer requests 999 shovels from the Mansion's computer system, causing it to crash and release the party. Berdly, Susie, Kris, and Lancer then meet up, but Lancer starts freezing because he is in an incompatible Dark World. Susie tries and fails to heal him. The party then encounters some puzzles, which Berdly attempts and fails to solve. After Kris successfully solves the puzzles, Susie and Berdly argue, and consequently Berdly reveals that he isn't actually that smart and is only successful thanks to Noelle.
The party then enters a dining room being prepared by Swatchlings and Ralsei. As soon as they meet, Susie asks Ralsei to heal Lancer. Ralsei explains that he can't. Because Lancer is foreign to this Dark World, the only solution is getting him to the Dark Fountain and out of the Cyber World.
Berdly, who knows the Mansion well since he had toured it while serving Queen, then comes up with a plan. Kris and Ralsei would distract Queen, while Susie would take a shortcut (located on the third floor) to the roof and transfer Noelle to Berdly. Berdly would then bring Noelle to a safe hiding spot, and the party would defeat Queen together.
Berdly then splits up from the group, and the party enters the Mansion's foyer. The party proceeds up the hallways, meeting new enemies and new traps. At the second floor, the party stumbles upon the Mansion's kitchen, which was dysfunctional due to a vermin infestation. They then defeat Mauswheel, making the eastern dining room accessible. After getting through the dining room with the help of the Annoying Dog, the party reaches the third floor.
After encountering Berdly again, Berdly and Susie split up, taking the shortcut to the roof. Berdly refers to Ralsei as Kris's lackey, and Ralsei asks about this. Kris and Ralsei stumble upon an acid lake, and are forced to ride a swan-shaped boat that can cross the lake. However, the swan is then stopped by a giant hand, forcing Kris and Ralsei to solve some puzzles and fight some enemies to move the hand away. After continuing their ride, Ralsei talks to Kris about what friends mean to him and how he was nervous about first impressions, even going as far to hide his face.
Kris and Ralsei then encounter Rouxls Kaard, who reveals the Thrash Machine from Chapter 1 as his secret weapon and attacks the party. The party plays a minigame against Rouxls if they wish to spare him, but no matter the outcome, Rouxls, just like Lancer, turns into a statue right after the battle. The acid lake ends with a camera making a photograph of the party, where Kris can choose to hug Ralsei, make a rude gesture, show a peace sign or not pose at all.
After leaving the acid lake, Kris can choose to watch Susie rescue Noelle, a scene in which Kris's choices do not alter the dialogue at all. Werewires enter Noelle's room but Susie pacifies them. Susie and Noelle then have a conversation about how the Dark World is actually a dream and why the "real" Susie does not pick on Noelle. Noelle asks Susie to wake her up, but she avoids it by asking Noelle to hang out some more. They jump on a Ferris Wheel with heart-shaped cabins to the east of Noelle's room and talk about Noelle's interest in scary things, Susie's Susiezilla fantasies and her tail. After getting off the Ferris Wheel they meet Berdly, who descends from the sky with a rose in his mouth. Susie, having accomplished her task, jumps away, and Berdly explains to Noelle that, despite valuing their mutual friendship, he had discovered that he is actually in love with Susie. The cutscene ends with Noelle angrily shouting at Berdly while she shakes him.
Susie falls down onto Ralsei and the party proceeds to the roof where they meet Queen. She reveals that she intercepted Noelle and Berdly, plugging the latter into control wires. The party battles against both the controlled Berdly and Queen. After the party successfully depletes Queen's HP or loosens Berdly's control wires, Berdly manages to break free, and Queen flees the scene.
Queen is then seen monologuing about the Roaring Knight, creation of the Fountains, and determination while trying to convince Noelle to be her servant. The party enters the scene and Queen flees, only to return with her giant robot previously present in the middle of the mansion, throwing the party members off the roof. Berdly catches the party using a roller coaster, and combines the party members, Sweet Cap'n Cakes, Rouxls Kaard, the Berdly statue, recruited enemies, and the Thrash Machine to make "the ultimate group project," the Thrash Robot. The robot enters a boss fight against Queen's final form, GIGA Queen, which works similarly to the arcade game from Cyber Field.
At the end of the fight, Queen uses her final attack, but the attack backfires, almost knocking her out. But, as she is about to be defeated, she uses her last ability, detachable hands, to knock the party out and capture the main party members: Susie, Kris and Ralsei. Queen threatens to kill the party if Noelle does not use the pin she gave her to tear new Dark Fountains. Noelle refuses, telling Queen she cannot be happy if controlled by her, and Queen tells her to choose the world that makes her happy before she runs out of battery.
Shortly afterwards Berdly comes in. After some dialogue, Berdly, Noelle and Susie begin to think if the Dark World is actually better than the Light World, and Berdly starts concentrating his force to create more Fountains. However, Ralsei quickly interrupts them, telling them about The Roaring. Queen wakes up, realizing that she was being misled by the Knight and releasing the party members.
Kris and Susie then seal the second Dark Fountain, waking the party up inside the Computer Room in the Library.
In the Snowgrave Route, the party isn't captured by Queen, and they enter through the main entrance. Most of the pathways are blocked, with Rouxls Kaard fully turned to stone in front of the Color Cafe, blocking it. The doorframe is also not active, preventing backtracking. Entrance to the hall with the basement door is open, and interacting with the door makes Spamton exclaim how he is preparing a new body. The stairway to the second floor is open from the start, and the paintings in the second floor are replaced with Spamton's face, no longer shooting flames at the party. Mauswheel attacks the party here. The kitchen is locked as well, and the dining area has a maze full of Pipis, which trigger a battle if stepped on. After exiting the maze, there are more minigames involving Pipis. At the end of the floor, the party encounters Tasque Manager, initiating battle on the spot. At the third floor, they can simply skip to the roof using the hidden passageway.
At the roof, Susie enters Noelle's room, but leaves shortly after without her. There is no battle with Queen in her battle area. Like in a normal route, Queen can then be seen monologuing about the Knight, but explains how Noelle is in no condition to assist her now, and that she will awaken to "a darker, darker dream". She then boards her robot, intending to get Kris or Susie to help her erect a new Dark Fountain, but Ralsei immediately cuts her off and explains The Roaring to her, skipping the GIGA Queen fight as well. Susie and Ralsei split up from Kris, who proceeds to the fountain alone, and is intercepted by Spamton NEO, who wants to rule the Cyber World himself, initiating the final boss battle of the route.
There are guest rooms at the beginning of the hallway, which serve as prison cells for the party. All rooms are tied to Queen's system, allowing her to observe and talk with the prisoners in the cells. There is also a communication device, allowing Susie to communicate with Kris during Queen's monologue. Items in these rooms are based on web searches of the guests present in them:
- Kris's room
- Dozens of bath bombs — "diy bath bomb"
- Jelly in shape of various knives — "sharpest kitchen knife jello"
- Jongler's Book on Jongleing And Other Tricks — "how to use magic"
- Calendar with college vacation circled — "summer vacation college when"
- VHS explaining how to install a video game piano — "video game piano tutorial"
- Two pixellized Toriels playing the piano — "video game piano tutoriel"
- Susie's room
- A warm painted egg — "can harboil eggs hatch"
- A vacuum-powered book — "LOSERS!!!! THIS LIBRARBY SUCKS!!!!"
- Copy of an online skateboarding game — "cool skatebord game online"
- A skateboarding game where the AI is cheating — "cool skateboard game online CHEAT"
- A glitched-out hologram — "FJFIrejirioEUEUROEI4378789"
After interacting with the computer in Kris's room, it appears to be overloaded due to a Dozens and Dozens of Shovels attack. Other rooms present in the guest hall are Berdly's, Catty's, Jockington's and Asriel's.
The hallway outside the prison cells leads to puzzles where Kris must move Darkbulbs to fill in marked space on the ground with dark. The first puzzle has a shape of Queen's face, the second is in the shape of a triangle and the third is a simple square. After solving the third puzzle, a cutscene with Berdly plays. As the party leaves the final room, the other two rooms are "unceremoniously" removed and the solutions to all the puzzles are projected onto the floor. The Hacker can appear in the room after coming back to it, he says he is trying to datamine the puzzle answers but only manages to get a child's toy to play a beep version of nursery rhyme music.
Eventually, the hallway leads to a dining hall, where Swatchlings (and Ralsei) are preparing for a dinner. Susie asks Ralsei to heal Lancer, and after explaining he cannot, the party can move Lancer's statue to the left or right. Moving him to the leftmost position puts him in a position for dinner.
The entrance to the Castle contains a Warp Door, some bookshelves, a SAVE Point, and the Color Cafe, where Swatch is the shopkeeper. Butler Juice, Spaghetti Code, B.ShotBowtie, and the Royal Pin are available as items in the shop. The legs of Queen's giant robot can be seen at the wall of the giant staircase.
Later on, shortcuts behind the two bookshelves in this area leading to higher floors, as well as a shortcut to the hall leading to the basement, can be unlocked.
Floor 1F is filled with Queen's paintings, which shoot fireballs at the party. There are also some monitors, belonging to Queen's system, and lots of pottery on the floor, which break when walked over, making Swatchlings chase the party to fight them. Among the paintings which shoot fireballs there are also paintings which if interacted with open paths so the party can proceed, with one of these flipping another painting so it becomes a treasure chest with a Revive Mint. Most paintings of Queen make facial expressions of laughing when the slide whistle in the background music (Pandora Palace, 0:19-0:20) plays.
Later on, the party enters a room with 4 portraits of Tasques, numbered from 1 to 4 and named "Task" (top), "Tasq" (bottom), "Tasc" (left) and "Timberly" (right). The party then encounters the Tasque Manager, who asks the party a couple of questions, to help her "organize the paintings":
- which painting comes first alphabetically (with the answer being "left," or "Tasc"),
- which painting comes third alphabetically (with the answer being "bottom," or "Tasq"), and
- which one is the fourth (but not alphabetically, so the answer is "left," or "Tasc").
The battle is then initiated with her and two Tasques by her side. If the questions were answered correctly, she is at 100% mercy and immediately spareable if the party wishes to.
The party can then proceed to the second floor through a teacup ride, this time with orbs which, if hit, damage party members who hit them.
To the right of the teacup ride to the second floor is another car puzzle and a room with statues behind it, in which the party can find an armor piece, Chain Mail. If all three checkmarks were collected for Hacker, he walks over that room and his head, normally a cursor arrow, turns into a pointing hand in the part of the room where the party can unlock a passage to the hall where the door to the basement is. After unlocking it, the party can open a shortcut to the mansion entrance to use a Warp Door, or proceed to unlock the lock between the mansion entrance and basement door using a KeyGen, an item sold by Spamton. Returning to the car puzzle, a mouse-induced traffic jam can be seen.
The party enters the second floor in Dinner Party Section A which contains several NPCs including an NPC Swatchling, and Nubert with Mousemillian At the end of the room is a puzzle with three silver platters, one of which contains the key out of the room. After the key is shown the platters are covered and spun around in another simple shell game. Opening the correct platter reveals the key and unlocks the door into the next room. Opening the wrong platter makes the bombs explode and fire bullets at the party, with the platter returning afterwards.
In the following rooms are more fire shooting paintings and paintings with switches that must be pressed to reveal the way forward. The switch on the left side of the room reveals a route to the dysfunctional kitchen where NPC Swatchlings are hiding from vermin while the switch on the right reveals a door to the dining hall which is initially locked. The western exit out of this room leads to the shortcut to 1F, which is locked until the party solves another platter puzzle similar to that in the previous room. Entering the kitchen and fighting the Mauswheel inside unlocks the door leading into the dining hall. The Swatchlings in the kitchen are also grateful to the party for dealing with the vermin.
The dining hall begins as a large empty room but once crossed Queen uses the mansion's system to reveal that the next key is hidden beneath one of the 48 platters she has placed in the room. Searching any three platters causes the puzzle to appear to reset. The key cannot be found regardless of how many times it is searched for. The only way to progress is by examining the platter on the table with the Annoying Dog hiding under. The dog then blindly charges across the room with the party on top of the table he is carrying. Steering the table as it charges across the room knocking over a large number of platters, it is possible to find the key, which is broken shortly afterwards by the dog. The party then exits the room by charging at the door. After charging down a short hallway crowded with Swatchlings the table crashes into the wall and the way back is blocked.
To the right of that point is a room with the "supreme table", a stack of several tables, which the party cannot use without the "Throne of the Gods", a stack of chairs. Behind the supreme table lies a royal water bottle, similar to the one King uses in his Castle Town jail cell.
In the left hall are Swatchlings guarding wheeled pottery. If any of the vases are broken the Swatchlings immediately turn hostile. To the north, there is an optional room with silver platters which, if inspected, will turn into spaceships and fly around the room attacking the party. After all the platters are inspected the spaceships leave and the last platter reveals a chest with a Glowshard. Through the hall, more paintings that shoot fire line the walls with part of Queen's giant robot being visible at one point. Past those obstacles is the way to the teacup ride to the third floor, during which there are blue orbs which bounce off the teacup platform, hurting the party if hit, and brown orbs, which cannot be avoided and do not do anything other than fill the teacups. Susie, however, seems to eat the brown orbs.
Berdly and Susie split from the party immediately after entering the floor, using a secret passageway hidden behind a bookshelf, leaving Ralsei and Kris alone. Swatchlings can be seen browsing a movie about how the Knight creates Dark Fountains, and how they created the Cyber World. The Berdly statue can then be encountered tucked into a toilet for Berdly statues, and an NPC can be seen waiting for the Berdly statue to finish after exiting the toilet. After that, there is an acid lake, which Kris and Ralsei traverse using a swan boat, initiating the main theme of the floor.
The party swims across the lake using the swan, talking with Queen through the mansion's displays, distracting her from Susie and Berdly. The swan is then stopped by a giant hand, and the party is forced to exit. To the north there is a fork with two roads, the left being the road through which the party returns to the starting point and the right leading to house puzzles. The party must change colors of tiny red and blue houses, which opens and closes certain paths through the tiny river islands. To the north of the first puzzle there is a tiny castle guarded by a Werewire, which, when defeated, leaves a chest with Revive Dust in it.
The last island of the puzzle gives the party an option to go north or east. To the east of the puzzle, there is a room with a golden Queen statue that is sinking into acid (the same one from the statue that Berdly made), another house puzzle in which a Swatchling got stuck while doing house cleaning, and a house which makes another vase fall from above to the bridge that the party must cross on their way back. The Swatchling starts chasing after the party after they break the vase; however, they let the party go since they freed them from the house puzzle.
To the north of the island in the previous puzzle, there is a combination of a mouse and house puzzle: mice need to enter two houses and turn at every crossing, and the party needs to adjust the crossings using houses so the mice enter their houses. There are also two unreachable treasure chests in this room; if mice are led into the treasure chests, they receive the treasure, and Mousemillian complains about how every mouse will be wearing a wig soon when interacted with next time. After leading the mice into houses, a bridge to the next room raises. The next room contains a lever, which makes another giant hand come out of the acid lake to high-five with another hand, freeing the way for the party's swan to proceed.
Later on, the party is stopped yet again by Rouxls Kaard, who initiates a battle riding the Thrash Machine. Rouxls's bottom half turns to stone after the battle, and the party proceeds to the roof.
The ascent to the roof contains a staircase down to the lower floors of the Mansion, an encounter with Werewerewire, Noelle's room and Trashy reminding the party to recruit everyone. Inside Noelle's room, the party can find an ICE-E statue (from a search query of "is ice e real cryptid") and a Susie-like statue, and steal them for their rooms. Most of her search results could not be made into objects, so they are stored in catalogues.
After traversing an electric puzzle, the party encounters Queen, initiating a fight against her and Berdly (who is plugged by her control wires). The final boss fight against the GIGA Queen takes place after the party is thrown off the roof, and the Dark Fountain is located here as well.
On the Snowgrave Route, a vending machine with CD Bagels can be found in front of the entrance to the Dark Fountain.
The basement can only be entered by Kris alone. Upon entering the basement, Kris finds a long hallway followed by stairs. Continuing brings them to a save point, with a forcefield to the east and a hallway to the west. Entering the hallway to the west, there are two easily avoidable Wall Plugs, with two entrances to rooms containing empty chests and a chest with 1 D$. At the end of the hallway is a door leading to a room where Kris has to walk back and forth until they find a teacup ride (seeing a pair of eyes with a sinister grin in the process) leading downward to a switch that deactivates the forcefield. Kris rides the teacups back up, dodging a significant quantity of falling orbs. Returning to the save point, Kris can continue down the hallway leading to the Empty Disk. Upon returning to Spamton, this disk is changed into a Loaded Disk, which enables the secret Spamton NEO boss fight when returned to the machine.
- The "sharpest kitchen knife jello" object in Kris's room is likely a reference to the YouTube video "sharpest jello kitchen knife in the world" by kiwami japan.
- The paintings of Queen are based on Mona Lisa, a famous painting by Leonardo Da Vinci. It differs with Queen's head and the hand gesture changed to an OK sign, which is used in the circle game.
- The Queen paintings that spit fire stop to laugh with Pandora Palace.
- Pandora Palace continues to play during encounters instead of changing to Rude Buster.
- On Floor 1F, if Kris respects the pottery, a vase lands on their head at the starting line. If they are able to balance the vase to the finish line, Susie congratulates them with a playful punch, leading to Kris dropping the pot.
- The vase does not fall if Kris moves up or down when they first receive it.
- If the group returns to the room with vases, Susie runs toward them and breaks the ones near Kris.
- The acid lake may be a reference to the RPGMaker game Off, as there are several places in the game where riding a swan boat across bodies of meat or plastic is essential for progressing the game.
- The melody of Acid Tunnel of Love is similar to A Stab of Happiness from Off.
- The "Susie-like statue" shares the same pose as the "MONSTER" in the Prophecy cutscene.
- Kris steals the "Susie-like statue" and the "ICE-E statue" without a prompt. This only happens upon investigating the objects.
- Sprite name for the statue sinking into acid is