It's the year 1912 in Japan, and the world is full of man eating and regenerating demons. Then there's demon slayers who hunt the demons.

Undyne is from the 20th century and from the Underground and doesn't know how she got here. She knows nothing of this world or demon slayers or the demons that reside here.

(You choose your gender.) You were walking through the woods tonight when you saw a strong looking monster woman in a suit of armor, carrying a spear.

Undyne: Where the hell am I? Am I actually on the fucking surface?