Chara falls down and befriends the royal family. On their birthday, they get a cookbook, which contains a recipe for a delicious pie. Chara teams up with Asriel to make this pie, but mistakes the cups of butter for the poisonous buttercup flower, which poisons Asgore. Toriel and Asgore have to make a tough decision and decide to hide Chara in fear that their accidental poisoning may be seen as attempted regicide. Asriel refuses to leave Chara alone, and goes into hiding with them.
One day, when Toriel and Asgore are walking back from a meeting, they're ambushed by a human who tries to kill them. They manage to shove Toriel off a ledge, and damage Asgore enough to put him in a coma. A ghost monster named Semi, who is a part of the Royal Council, comes to the rescue, slays the human, and takes their SOUL. Per regulation, the head of the Royal Guard, Gerson, takes over the trone. But he decides to take Semi along with him. As he sees benefit to having a monarch that is pretty much unkillable by human hands. Semi pushes for a law that would see every human who falls down as the enemy, and their SOUL be taken and delivered to the monarchy to break the barrier. Much to Gerson disagreement, this law is quickly passed. Semi establishes many reforms that change the Underground in many ways. Gerson's role is gradually pushed to a more constitutional one, as Semi is the one making most of the decisions.
One day, when the next heir to the throne is to be chosen, something unexpected happens. The heirs are the ghost monsters Mettaton and Napstablook, to keep with the tradition of having an unkillable monarch. Much to everyone's surprise, the cowardly, reserved Napstablook is crowned monarch instead of the boisterous and prideful Mettaton.
In present day, the human Frisk falls into the Underground, and are faced with one simple, yet terrifying fact. The monsters have thus far collected 6 human SOULs, and are yerning for the 7th. How will Frisk navigate though this hostile world when they in principle don't want to hurt anyone? Will they give in when push comes to shove, or will they strive to resolve conflicts pecefully?