Mystery Key

The rightmost house is unlockable.

The Mystery Key (JP: ナゾのカギ) is a miscellaneous item sold by Bratty and Catty. Bratty and Catty describe it as "????? Probably to someone's house LOL". It unlocks the house adjacent to Napstablook's home.

Additional uses

• [If the Mystery Key is used on the mysterious door, it reads the following text]
  • You used the Mystery Key. The door's lock clicks...
  • ... as you fail to fit the key into it.
  • Nothing happened.
• [If used during the fight against Mettaton EX, the following text displays]
  • You used the Mystery Key. Mettaton pretends it isn't there.

Location

Loading scripts

Main story

The room unlocked by the Mystery Key, containing six diaries.

Bratty and Catty mention how Mettaton said that being built was his own idea and how he considered Alphys to be an old friend. It is explained that the soul that Alphys put into Mettaton's body is, in fact, Napstablook's cousin and neighbor, who wanted to become corporeal but would never find the kind of body he was looking for. He hosted a "human fanclub", which only Alphys showed up to. She ended up sketching and showing Napstablook's cousin an illustration for a body "beyond his wildest dreams".

Trivia

• If Undyne and Papyrus are called while in the house, Papyrus dubs the ghost who Mettaton once was "HAPPSTABLOOK, THE HAPPY GHOST". Alongside this, the name of Mettaton's house in the game files is room_water_hapstablook.