You are Frisk, a genocidal human child.

After walking into Alphys' lab in Hotland, you entered into the True Lab after you found out the door was open, when it wasn't supposed to be...

After riding the elevator down, and walking around for a bit, you arrive at the DT Extractor room, and are greeted by Sans the Skeleton.

"heya." He speaks, his voice echoing. ". . ." "there's been a change of plans. there's a few things i'd like to talk about with'cha." He stated. ". . ." "kid. i'm tired. why not relax and take a load off? it's way easier." He suggested. "but we know that isn't gonna happen. you'll never be happy, will you?" He asked. "welp. you probably know what this machine does." He says, motioning to the DT Extractor. "and you can probably guess what i'm gonna do next." Suddenly, blue bones surround your body, making you unable to move without getting damaged. "you'll be dead where you stand." Sans threatens, as you're thrown into battle.

Sans stands behind the DT Extractor, having activated it using the control panel. He begins extracting the DETERMINATION from your SOUL, lessening your HP! After he's done, he throws a barrage of bone attacks and Gaster Blasters at you. By the end of it, you're left with only 22/32 HP. "is this what you want?" Sans asks, as you're allowed a turn.