So during the human-monster war, Asriel is killed before the monsters are trapped Underground. The monsters have split opinions on what they should do next. Some monsters, known as The Reprisal, want to collect seven human souls to escape before waging war on humanity.

Some monsters, known as The Redemption, want to spare humanity and try to find a different solution. A few monsters are neutral in the conflict. The two opposing sides, one led by Asgore (The Reprisal) and the other led by Toriel (The Redemption), wage war on each other. 

(List of monsters and what side they are on)

Toriel- The Redemption

Papyrus- The Redemption

Sans- The Redemption

Gaster- The Redemption

Asgore- The Reprisal

Undyne- The Reprisal

Mettaton- The Reprisal

(If you guys want a full list let me know)

Asgore isn't very enthusiastic about the war or killing humanity and wants to end the battle peacefully. Unfortunately things don't work out for Asgore as Undyne decides to go and kill Toriel, believing it will make the enemy weak. Undyne is successful but didn't realize that The Redemption would not back down easily.

Gaster decides to even the playing field by taking out Asgore, he succeeds and even destroys Undyne's eye. He soon becomes the new leader of The Redemption and Undyne becomes the leader of The Reprisal.

A few months later a human, Chara, falls into the Underground. They are injured from the fall and calls out for help. But nobody comes. Chara grabs a nearby stick and uses as support for their injured leg.

Soon they meet their first monster, Papyrus. Papyrus takes Chara to the palace to be treated. While Chara recovers they meet Sans and Gaster and are told about the war.