Undertale Story
A human child falls into the Underground and meets Flowey, a sentient flower who teaches the player the game's mechanics and encourages them to gain "EXP" by killing monsters to increase their "LV," or "LOVE." When Flowey tries to kill the human and take its soul, Toriel, a motherly goat-like monster, saves the human and teaches him how to solve puzzles and survive conflict in the Underground without killing. She plans to adopt the human in order to protect them from Asgore Dreemurr, the Underground's king.
Searching
After leaving Toriel, the human searches for Asgore's castle, which houses the barrier to the surface world. They come across a number of monsters, including the skeleton brothers Sans and Papyrus, who serve as sentinels for the Snowdin forest; Undyne, the head of the royal guard; Alphys, the kingdom's royal scientist; and Mettaton, a robotic television host created by Alphys. The majority of them are fought, with the human having the option of killing them or sparing and even befriending them.
During their journey, the human discovers the origins of the human-monster conflict. Asriel, Asgore and Toriel's son, befriended and eventually adopted the first child who fell into the Underground. The child died one day after eating poisonous flowers. Asriel returned the body to the humans, they attacked him and killed him, prompting Asgore to declare war. Asgore is now trying to break through the barrier, which requires him to gather seven human souls, of which he already has six.
Don’t Kill The Monsters
The outcome of the game is determined by how the player handles monster encounters. If the player completes the game without killing any monsters (or kills some but not all monsters), the human will arrive at Asgore's castle and discover that a monster's soul is also required to cross the barrier, forcing them to fight Asgore. Sans intervenes before the human and reveals that the human's "LOVE" stands for "level of violence" and "EXP" stands for "execution points." Sans evaluates humans based on their total "LOVE" and "EXP."
Asgore is fighting the human, but Flowey intervenes, killing Asgore and stealing the human souls. The human defeats Flowey with the help of the rebelling souls falls unconscious, and awakens on the human side of the barrier; they receive a phone call from Sans detailing the state of the Underground following the human's departure. The "Neutral" ending has several different epilogue phone calls depending on which monsters were killed and which ones were spared.
Okay, We Can Kill Some
The so-called "Pacifist" route can be completed if the player kills no monsters after completing a "Neutral" ending. Alphys' experiments have created Flowey, who is revealed to be a reincarnation of Asriel. Before the fight with Asgore, Toriel intervenes and is joined by the other monsters the human befriended. Flowey ambushes the group, reclaiming all of the human souls as well as the souls of all the monsters to fight the human in an older Asriel form. During the fight, the human connects with their friends and eventually wins.
Before leaving, Asriel reverts to his child form, destroys the barrier, and apologizes to the others. The human falls unconscious and is awoken by the presence of their friends, who know the human's name: Frisk. On the surface, the monsters reintegrate with the humans, and Frisk has the option of adopting Toriel as their mother.
No Mercy
If the player kills all monsters, a third ending is known as "No Mercy" or "Genocide" occurs. Sans tries and fails to stop Frisk when he arrives at Asgore's castle. In an attempt to gain mercy, Flowey kills Asgore but is then killed by Frisk. The fallen child Chara, whose body Asriel attempted to return, appears and destroys the universe. Frisk must give their soul to Chara to restore the universe, which will permanently alter all subsequent pacifist runs, to enable further replays of the game.