The Mysterious Door is an area found in Snowdin Forest, south of the right side of the room with the icy X's and O's puzzle. The door, which bears the Delta Rune, is surrounded by glowing mushrooms and floating particles.
To open the door, the protagonist must dodge all the names of the Undertale Kickstarter backers in the True Pacifist Ending Credits. There are other ways to open the door, such as editing the SAVE data.
The room before Dog Room is a long and narrow vertical hallway that opens up into a small, square room with the door itself. In the small room, there are four glowing mushrooms that can be turned on or off, but otherwise do nothing else.
Beyond the door lies the "developer room" which includes the Annoying Dog who represents Toby Fox, a computer open to bad posts that the dog has made, a jigsaw puzzle, the rope that was likely used in Papyrus's Gauntlet of Deadly Terror, a "text-to-speech" converter that the dog somehow used to program the game, and a dog-shaped husk of Dog Residue.
- [When interacting with the game case]
- (By barking with text-to-speech on, the dog accidentally programmed a whole game)
- (Hmmm... seems fine.)
- [When interacting with the rope]
- (Looks like this dog does all its own stunts.)
- [When interacting with the puzzle]
- (Finally, the pieces of the puzzle are coming together.)
- (The jigsaw puzzle.)
- [When interacting with the dog to the left]
- (You tried to pet the dog, but it collapsed into a pile of fluff...)
- (Seems like it was actually just a dog-shaped husk of dog residue.)
- (It's a pile of dog residue.) [When interacting again]
- [When interacting with the computer]
- (Looks like this dog makes a lot of bad posts on-line.)
- (Text to speech is on.)
- (Seems like the dog barks, and it translates to these bad posts)
- (It doesn't really know what it's saying.)
- (Seems like most bad posts on-line, are actually this dog.)
- (Thinking about this...)
- (Brings you relief.)
- [When interacting with the dog in the middle]
- (It's a small white dog.)
- (It's fast asleep...)
- (Fight the dog?)
- [If "No" is selected]
- (Let sleeping dogs lie, instead of fighting them.)
- (That's how the saying goes.)
- [If "Yes" is selected]
- (Can't fight the dog.)
- (Seems like the fabric it's sleeping on has too many holes in it.)
- (Seems like the dog needs to "patch" the fabric.)
- (Then you can fight the dog.)
- (... maybe.)
- (Upon closer examination, the holes in the fabric seem to be growing.)
- (Might take a while for the dog to fix all of them.)
- (Dogs aren't usually very good at knitting.)
- (A crocheting dog is out of the question.)
- The dog made up of residue that then falls apart may be a reference to page 932 of the webcomic Homestuck.
- The piece of fabric that the Annoying Dog is lying on states that "the dog needs to 'patch' the fabric." This is most likely referring to the actual game itself, as Toby Fox needs to patch the game up.
- If the Mystery Key is used when facing the door, a click is heard, but the door does not open.
- The music that plays in the room,
mus_dogroom.ogg, cannot be found in the Undertale Sountrack.
- This is very likely a reference to the Silent Hill 2 dog ending.
- If the room is entered during the epilogue, the music continues to play when leaving the room.
- The door remains open, even after a True Reset. However, the door is reset when completing the Genocide Route.
- If the protagonist interacts with a floating particle as if it were an NPC, a special sound effect plays.
- You used the Mystery Key. The door's lock clicks... ... as you fail to fit the key into it. Nothing happened. - Flavor Text