The Ruins is the tutorial area of Undertale as well as the entire setting of the demo. The protagonist enters the Underground by falling from a giant hole in the ground on Mt. Ebott into the Ruins before the beginning of the game.
The game begins in the Ruins, with the protagonist standing on a sunlit bed of Golden Flowers.
After the protagonist is attacked by Flowey, Toriel takes the protagonist through the Ruins, which is made up of several connecting hallways and rooms, many of which have puzzles and monsters. Partway through, Toriel asks the protagonist to wait where they are, so she can go ahead and finish something. However, the protagonist continues through the Ruins, and traverses to the end of the Ruins to Toriel's home. While navigating the Ruins, the protagonist meets Napstablook for the first time and has the option of supporting the Spider Bake Sale. While they go through the Ruins, Toriel calls them multiple times.
At the end of the Ruins, the protagonist meets up again with Toriel. Toriel brings the protagonist to her home and introduces them to their own room. If the protagonist sleeps in the bed, Toriel leaves a Butterscotch-Cinnamon Pie at the foot of their bed. Afterward, she departs, telling the protagonist to "make [themself] at home!" However, the protagonist proceeds to pester Toriel about leaving, prompting Toriel to rush out of her armchair, and rush downstairs. Upon being followed, Toriel tells the protagonist that if they leave the Ruins, Asgore will kill them, and urges them to return. However, the protagonist continues to pursue Toriel, eventually having to fight Toriel to leave the Ruins.
After the protagonist deals with Toriel, the protagonist encounters Flowey, who judges the protagonist on their actions in the Ruins and/or from previous runs - excluding a Genocide Route - done without a True Reset. Afterward, a door behind Flowey becomes accessible, letting the protagonist proceed into the rest of the Underground.
After the monster population was banished to the Underground by the humans, they made their first residence in what is now known as the Ruins. Asgore, King of the Monsters named this area "Home." When the First Human fell into the Ruins, they were befriended by Asriel Dreemurr and were adopted by Asriel's parents, Toriel and Asgore. After the First Human's and Asriel's death and Asgore's subsequent declaration of war against humanity, Toriel took the First Human's body with her to the Ruins, where she buried the body. Eventually, monsters began to head back to the barrier, where they set plans to create a new home for monsters.
Entrance to Leaf Pile
The first room of the Ruins consists of a pile of golden flowers, onto which the protagonist initially falls, a short hallway, and a large doorway embellished with the Delta Rune. The golden flowers in this room can only be examined after meeting Flowey. If Toriel is spared at the end of the Ruins, she can be found here, tending to the flowers.
The second room is empty other than a small illuminated patch of grass, which is where Flowey is first encountered, along with another doorway that is nearly identical to the first door the protagonist walks through.
The following room is the first that contains the purple brick decor that characterizes most of the Ruins. This first room contains a SAVE Point. Following this room, there lies the Middle Road Puzzle, followed by the Switch Puzzle, and then a room that contains the Dummy. Following this room is yet another puzzle, the Blueprint Puzzle, containing markings on one side of the floor that are intended to reveal the correct path through a series of spikes.
Following this room is an incredibly long hallway, at the end of which is a pillar. Here, Toriel instructs the protagonist to travel across the hallway without her guidance. After completing the "Independence Test," Toriel tells the protagonist to remain in the room and gives the protagonist a Cell Phone. Talking to the pillar before Toriel reveals herself gives the text "...", with Toriel's picture. If the protagonist waits in the room, Toriel calls them every five minutes. Transcripts of these conversations can be found here.
Leaf Pile to Mouse Hole
This path is mostly filled with simple puzzles and weak monsters such as Froggit and Whimsun. This is the first area where it is possible to encounter monsters. At the entrance to this room, a Froggit NPC informs the protagonist on how to fight and act, and ask them to show mercy to monsters that do not want to fight anymore. If any monsters are killed in this room, the Froggit disappears.
If the protagonist walks on the path north of the leaf pile, they find a small room with a single pedestal. The protagonist can take four (three in Hard Mode) pieces of Monster Candy from the pedestal before it falls over.
The path southeast of the leaf pile leads to a room with a 2x6 pattern of cracks in the floor. This is the first room where the protagonist can encounter Moldsmal. To the left and right of the cracks are two wide chutes. When the protagonist steps on the cracks, they fall into a small room directly below. In this room, only a leaf pile and two chutes are visible. When the protagonist walks up to the chutes on the lower floor, they are taken to chutes parallel in the room above. Climbing up the rightmost chute in the lower room allows the protagonist to bypass the cracks and move forward.
The next room contains the first rock puzzle.
Past the rock puzzle is the leaf puzzle. This room contains a large section of the floor that is covered with cracks, while the room below it contains a path through a pile of leaves. If this path is followed on the upper floor, the protagonist is able to walk on the cracked floor without falling through the cracks.
The next room contains the second rock puzzle.
Mouse Hole to Home
The room following the second boulder-pushing puzzle is a room containing a mouse hole, a SAVE Point, and a small table. Checking the table informs that cheese is stuck to the table due to being there so long. Checking the mouse hole causes a small squeak to be heard.
The right passage leads to two more passages: one going up, and one continuing right. Both paths are blocked by Napstablook. If Napstablook is interacted with, and the protagonist chooses to move them with force, an encounter is initiated. Following this encounter, Napstablook vanishes, allowing the protagonist to proceed. If the kill counter has been exhausted before meeting Napstablook, they disappear when approached.
The room right of Napstablook leads to two spiderwebs and a sign. The sign informs the protagonist of the Spider Bake Sale, which is held in the two nearby spiderwebs. The smaller web exchanges 7 G for one Spider Donut. The larger web takes 18G and gives one Spider Cider in return.
The path north of Napstablook leads to a large room with three Froggits, who inform the protagonist of various aspects in the game. This is the first room where Loox, Migosp, and Vegetoid can be encountered. Information regarding the Froggit's dialogue can be found here. The room right of the Froggits contains the The One Switch Puzzle. The next room contains the Rotating Puzzle, which consists of the same room rotated around four times. Each of these rooms contains three pillars, and three colored switches adjacent to them. Pressing the correct switches lowers spikes that block the way, allowing further advancement.
The following room is a crossroads. In this area, it is impossible to encounter any monsters. Here, the protagonist can find two paths. One leads north, while the other leads east.
The eastern path leads to a small room, which houses a single Froggit NPC, and a door leading to the north. Going through the north door near the Froggit leads to an overlook of Home. The Toy Knife can be found on the left side of this overlook.
Going through the north door at the crossroads leads to Toriel's Home. A large, leafless tree is found at the entrance to Toriel's home. Shortly after entering this room, Toriel approaches the tree, and attempt to call the protagonist on their cell phone. She stops upon noticing the protagonist, however. After inspecting their wounds, Toriel will ask the protagonist to accompany her up to the house. Near the doorway, a SAVE Point is visible.
Toriel's Home to Ruins Exit
The entrance of Toriel's Home contains three paths: a left path towards a living room, a right path towards a hallway, and a downwards staircase that leads to the Ruins Exit.
- When doing the second "rock pushing puzzle," one of the rocks does not want to be pushed (at first). If the protagonist ignores the other rocks required for the puzzle and instead keeps pushing the reluctant rock, the barrier still moves.
- This clarifies the sign in the room with the first rock puzzle, "Three out of four rocks recommend you push them."
- In the room with three visible Froggits, the fourth Froggit, which is ant-sized, can be found inside of one of the wall cracks. This frog is found by inspecting the northern wall in the room.
- If the protagonist talks to the third Froggit repeatedly, they can make it so monsters names no longer appear yellow when they can be spared. They can also switch it, so the names appear pink instead.
- If the protagonist has killed Toriel or fled from her during her battle, then sleeping in the bed again triggers the message "<Name>, please... Wake up! You are the future of humans and monsters..." and they wake up.
- After leaving the Ruins, the door leading to the Ruins is locked for the rest of the game and the only way for the door to open back up is to beat the final boss of the True Pacifist Route.
- Depending on the protagonist's HP when finding her home, Toriel's dialogue before telling the protagonist to enter her home varies.
- Full HP - "Not a scratch... Impressive! But still..."
- Less than full HP, but more than 2 - "There, there, I will heal you."
- 2 HP or less - "Who did this to you? You will get an apology."