* Undertale takes place in the Underground, a realm where monsters, once equal to humans, were banished to after war broke out between them.
* The Underground is sealed from the surface world by an imperfect magic barrier, the only point of entry being at Mount Ebott.
* A human child falls into the Underground and encounters Flowey, a sentient flower who teaches them the game's mechanics and encourages them to raise their "LV", or "LOVE", by gaining "EXP" through killing monsters.
* When Flowey attempts to murder the human to take their soul for himself, the human is rescued by Toriel, a motherly goat-like monster, who teaches the human to solve puzzles and survive conflict in the Underground without killing.
* She intends to adopt the human, wanting to protect them from Asgore Dreemurr, the king of the Underground.

* The human eventually leaves Toriel to search for Asgore's castle, which contains the barrier leading to the surface world.
* Along the way, the human encounters several monsters, including: the skeletons Sans and Papyrus, two brothers who act as sentries for the Underground; Undyne, the head of the royal guard; Alphys, the kingdom's royal scientist; and Mettaton, a robotic television host Alphys created.
* Some of them are fought, with the human having to choose whether to kill them or to show mercy; should the human spare them, they become friends.
* During their travels, the human learns the cause of the war between humans and monsters.
* Asriel, the son of Asgore and Toriel, befriended a human child who fell into the Underground and was adopted by Asgore and Toriel.
* One day, the child was fatally poisoned by some flowers.
* Asriel absorbed the child's soul, which allowed him to cross through the barrier.
* When Asriel returned the child's body to the humans, they attacked and killed him.
* Asgore now seeks to break the barrier, which requires him to collect seven human souls, of which he has six.

* The game's ending depends on how the player resolved encounters with monsters.
* If the player killed some but not all monsters, the human arrives at Asgore's castle and learns that they also need a monster's soul to cross the barrier, forcing them to fight Asgore.
* Sans stops the human before their confrontation, revealing that the human's "LOVE" is an acronym for "Level Of ViolencE" and "EXP" for "EXecution Points."
* Sans judges the human based on the combined resolution of the encounters.
* The human fights Asgore, but Flowey interrupts them, killing Asgore and stealing the human souls, becoming a monstrous abomination.
* With the aid of the rebelling souls, the human defeats Flowey, falls unconscious, and awakens on the human side of the barrier; they receive a phone call from Sans, explaining the state of the Underground after the human's departure.

* If the player instead kills no monsters, as well as having completed a previous Neutral playthrough of the game, the player gains access to the Pacifist Route, returning the player to immediately before they fight Asgore.
* If they haven't done so already, the player goes about befriending Papyrus and Undyne, then goes to cheer up Alphys.
* Flowey is revealed to be a reincarnation of Asriel, created as part of Alphys's experiments.
* Toriel intervenes before the human fights Asgore, and is joined by the other monsters the human has befriended.
* Flowey ambushes the group, using the souls of all the monsters to take an older Asriel's form to fight the human.
* The human connects with their new friends during the fight, eventually triumphing.
* Asriel reverts to his child form, destroys the barrier, and expresses his remorse to the others before leaving.
* The human falls unconscious, and is awoken to see their friends surrounding them, with the knowledge of the human's name - Frisk.
* The monsters reintegrate with the humans on the surface, while Frisk has the option of accepting Toriel as their adoptive mother.