After Flowey broke the reset system to stop Chara, leaving only a battered Sans behind, the princesses find the barrier and remove it. Things can only get more humerus from here.

We'll be following Sans as he makes a skele-ton of puns, starts a worldwide prank war, drink ketchup, and scam ponies into eating his 'special' hot dogs. Though, an evil presence is following close behind him, one he knows all too well.