"Temperature is dropping... I'm not sure if I can... See this ever stopping.... ...Shaking hands with the dark parts of my thoughts... No, you are all that I've got, no. Don't forget ab-b-b-b-bout me. Don't forget ab-b-b-b-bout me. Even when I doubt you... I'm no good without you, no, no, no, no, no."

-- Doubt, Twenty One Pilots (main inspiration)

Underdoubt is an Undertale AU created by User:Aidanthehedgehogisawesome. It centers completely around Gaster within the SPACELESS.


Once upon a time, there was a scientist name Gaster.

He was a brilliant man who was often deemed the smartest MONSTER in the Underground.

Unrivaled in his generosity and skill, many thought the doctor would never leave them.

But, a jealous assistant, ALPHYS, organised a way to get rid of him.

Sometime in the night, Gaster was shoved into his creation; the CORE.

There was an uproar in the Underground, and, eventually, ALPHYS was elected new Royal Scientist.

But, even when they doubted Gaster, they'd not forget him. They're no good without him.


The main plot shows Gaster documenting the different creatures of the SPACELESS. He shows sorrow and optimism as he goes through, and supposedly reaches his universe at some point in time.


  • Gaster : Gaster is the main hero of the tale. He wears a large lab coat, and is a tall, lanky figure with large hands and a larger head. He posses a horn and a 'broken' looking face.
  • Alphys : Alphys is the main villain of the AU. Though she only appears once, and is only mentioned for the time being.
  • Smolbean : Smolbean is an insectoid, canine-esque mushroom creature that is discovered by Gaster. Smolbean becomes attached to Gaster, and accompanies him through the remaining of his journey.


  • Like, literally the entire thing is based on a single song.
  • Smolbean is also based on twenty one pilots.