The rightmost house is unlockable.

The Mystery Key is an 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 key, the lock clicks..... as the key failed to fit in the hole.

If used during the fight against Mettaton EX, the following text displays:

You used the Mystery Key. Mettaton pretends it isn't there.

Main Story

Mystery key room

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".

Diary Entries

  • Dear Diary: Shyren's sister "fell down" recently. It's sad. Without her sister to speak for her... she's become more reclusive than ever. So I reached out to her, and told her... that she, Blooky, and I should all perform together sometime. She seemed to like that idea.
  • Dearer Diary: I like to buy a new diary for every entry I make. I love to collect diaries.
  • Dearest Diary: Our cousin left the farm to become a training dummy. That leaves just Blooky and I. Blooky asked me if I was going to try to become corporeal, too. They sounded so... resigned... come on, Blooky. You know I'd never leave you behind. And besides... I'd never find the kind of body I'm looking for, anyway.
  • My Darling Diary: I met someone... interesting today. Last week I posted that advertisement for my Human Fanclub. Today was our first meeting. Only one other person came. Honestly, she's a dork. And she's obsessed with these awful cartoons. But she's kind of funny, too.... I want to see her again.
  • Diary... My dear: My diary collection is going fabulously. I have like five now.
  • Dear Diary: She surprised me with something today. Sketches of a body that she wants to create for me... a form beyond my wildest fantasies. In a form like that, I could finally feel like... "myself." After all, there's no way I can be a star the way I am now. Sorry, Blooky. My dreams can't wait for anyone...


  • 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.
  • Even though the Mystery Key can be obtained for free on a Genocide Route, the house is inaccessible, because there's no way to go back to the Waterfall once the protagonist goes sufficiently far into Hotland.

Consumables Monster Candy • Bandage • Spider Donut • Spider Cider • Butterscotch Pie / Snail Pie
Snowman Piece • Bisicle / Unisicle • Cinnamon Bunny • Temmie Flakes • Abandoned Quiche
Dog Salad • Astronaut Food • Instant Noodles • Crab Apple • Hot Dog...? / Hot Cat
Glamburger • Sea Tea • Starfait • Legendary Hero • Bad Memory • Last Dream • Popato Chisps
Nice Cream • Junk Food • Steak in the Shape of Mettaton's Face • Hush Puppy
Weapons Stick • Toy Knife • Tough Glove • Ballet Shoes • Torn Notebook
Burnt Pan • Empty Gun • Worn Dagger / Real Knife
Armor Bandage • Faded Ribbon • Manly Bandanna • Old Tutu • Cloudy Glasses
Stained Apron • Cowboy Hat • Heart Locket / The Locket • Temmie Armor
Miscellaneous Punch Card • Annoying Dog • Dog Residue • Undyne's Letter (EX) • Mystery Key • Silver Key
Cut Content Croquet Roll • Rock Candy • Pumpkin Rings • Stoic Onion • Ghost Fruit • Puppydough Icecream
Purchasable items
Spider Bake Sale Spider Donut • Spider Cider
Nice Cream Guy Nice Cream
Snowdin Shopkeeper Tough Glove • Manly Bandanna • Bisicle • Cinnamon Bunny
Gerson Crab Apple • Sea Tea • Cloudy Glasses • Torn Notebook
Tem Shop Temmie Flakes • Temmie Armor • tem pay 4 colleg
Sans Hot Dog...?
Bratty and Catty Junk Food • Empty Gun • Cowboy Hat • Mystery Key
Burgerpants Starfait • Glamburger • Legendary Hero • Steak in the Shape of Mettaton's Face
Vending Machine Popato Chisps