Plot
Undertale takes place in the Underground, a realm to where monsters, once equal to humans, were banished 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 Snowdin forest; 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 may choose to 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 the first human child who fell into the Underground and was adopted by Asgore and Toriel. One day, the child killed themselves by eating poisonous flowers. When Asriel returned their body to the humans, they attacked and fatally wounded him, resulting in Asgore declaring war. 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. 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 and has completed a previous playthrough of the game,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.
Another ending ensues if the player kills all monsters,in which Frisk becomes influenced by Chara, the fallen human child whose body Asriel attempted to return. When Frisk reaches Asgore's castle, Sans attempts to stop them, but Frisk kills him. Flowey kills Asgore in an attempt to get mercy, but Frisk executes Flowey anyway. The fallen human child assumes control and, with or without the player's consent, destroys the universe. To enable further replays of the game, Frisk must first give their soul to the fallen human child in exchange for restoring the universe, which will permanently alter every subsequent Pacifist run.
Development