Uppertale is a fanmade sequel to the indie hit, "Undertale"
As the Pacifist ending happens, Frisk awakes in their bedroom, Toriel is making a butterscotch and cinnamon pie and tells Frisk to play with his/her friends. As Frisk leaves and heads for the fields, they encounter Sans, looking at trees for no reason.If you gained EXP, Sans will comment on this and says "Nobody kills on accident", and if you didn't gain EXP Sans asks Frisk to go to Grillby's with him. As they reach Grillby's, Sans talks to Frisk about timelines, no matter what you choose, he will decide you are the reason and asks you to not reset, after that Alphys will appear near the area back home and asks you to come with her. If you've gained EXP, she will appear in your neighborhood. You go to Alphys' house and enters her basement, it turns out that she built a time machine, you got 3 options "Hotland, Waterfall, or Snowdin" after time traveling, she will ask you to take the picture of Her and Undyne Smooching, you go to Asgore's castle on a time machine, and when you reach there, Flowey will see you and see the time machine, he begins a fight with you, and if you kill him, a true reset will be performed, if you managed to frustrate him, he will end the fight and get the six human souls as your friends do what they normally done in a pacifist Asgore fight, except without Asgore. Flowey takes your friends souls, along with every other monster, like normal, and then kills past Frisk and Asgore and takes there souls, after this happened, the game will close, reopening it will initiate a fight with Flowey and, after you beat him, he will turn the timeline back to normal. After this, you will return from the machine, and you will say to Alphys that you didn't take the picture, she says that she didn't want you to cause "All there was, was just that flower" after that your friends come and ask what happened, and after a long conversation, credits roll.
Genocide Route Edit
If you slay all the monsters in the field, you will when you meet Sans, initiate a fight with him, after killing him Papyrus will cry and be mad at you, you kill him as well, then Flowey appears, and you kill him as well, after that, Chara appears and destroys the world, doing a slash animation and destroying the game forever, no way to restore it, other than deleting the genocide file in appdata, or do a fresh reinstall.