Undertale is an indie 2D RPG video game released on PC in September 2015, on PS4 (and PS Vita) in August 2017, on the Nintendo Switch in September 2018, and on Xbox One (and Xbox Series X/S) on March 16, 2021. Developed by Toby Fox, the game follows a child who has fallen into the Underground on his journey back to the surface. The player has the choice of whether or not to kill all the monsters they meet (or just some) or spare all of them (or just some).
The choices the player makes will affect the outcome of the story. There are three different main endings: Neutral, Genocide, and True Pacificist. (There is also a fourth one called Hard Mode.) In the True Pacifist Route, the player kills no monsters and therefore gains no EXP/LOVE.
This ending is considered the true ending to the game because the player (named Frisk through this route) escapes the Undeground to the surface with everyone. On the surface, the monsters talk about trying to make friends with the humans and live among them in peace. Frisk may choose to stay with Toriel or go off on his own.
8 It Is Okay To Kill The Dummy
The whole point of the True Pacifist Route is to not kill anyone. In the game, EXP is gained after killing monsters. While in combat, only sparing, fleeing, or ACTs that end the battle is allowed.
However, the player can kill the Dummy in the Ruins, because it does not give any EXP. They can also attack Mad Mew Mew and FIGHT monsters in general as long as no one is murdered. (Accidents happen though, and the player can reload their save file if anything.)
7 Dating Papyrus Is Completely Necessary
In addition to not killing anyone, the player must also hang out with Papyrus. He is the brother of Sans and a flamboyant skeleton. They meet him in Snowdin Forest, where he has them complete various, silly puzzles.
After fighting him (and not murdering him) at the end of the level, they must accept his date invitation. If not offered one, the player can backtrack to his house in Snowdin. Befriending Papyrus is a requirement to the True Pacifist Route.
6 Throw Some Water On Undyne
Besides not killing Undyne (and the other monsters), the player must also save Undyne from heatstroke. Undyne is the fish-like monster that chases the player throughout the Waterfall level. She passes out from exhaustion at the end, if still alive.
After Undyne collapses, the player must grab a cup of water from the nearby water cooler and pour some all over her. They cannot leave the room before doing so or she will disappear and the True Pacifist Route becomes impossible!
5 Cooking With Chef Undyne
In the Waterfall area, and only if the player gained zero EXP thus far, is Undyne's House. Outside of it, Papyrus will be waiting to greet the player. Eventually, he leaves and Undyne has a drink with the player.
Additionally, she will make the player cook spaghetti with her (which goes horribly wrong and ends up in literal flames). Then, Undyne challenges the player to battle. (She cannot die and quits fighting them eventually). It is a requirement to the True Pacifist route to befriend Undyne.
4 Mettaton's Ratings Are Over 9,000
The player meets Mettaton in the Hotland as well as the CORE. He is a robot from the Lab, created by Alphys. Mettaton wants to be a star someday.
After the player flips his switch, Mettaton becomes Mettaton EX, a humanoid version, and engages in battle with them. If he takes damage, uses popular brand items, or ACTs, his ratings for the show will go up. The player must get Mettaton's ratings to 10,000 to subdue him successfully.
3 Defeating Asgore And Flowey (Without Violence)
During the finale of the game, the player meets Asgore. He is the current rule of the Underground and plans on killing seven humans to use their SOULs to destroy the barrier that is trapping him and all the other monsters in the Underground. Asgore is also the ex-husband of Toriel and the father of Asriel.
After defeating Asgore (but not killing him, of course) with the help of Toriel, Flowey, the main villain of the game, shows up again. Following his defeat (not death) while in his Photoshop Flowey/Omega Flowey/Flowey X form (because he absorbed six human SOULs), the player can spare him to complete the True Pacifist Route...almost.
If this was the player's first time playing the game, they must reload their savefile. If not, meaning they already completed the Neutral Route previously and therefore delivered Undyne's Letter and found the True Lab during this playthrough, they completed the True Pacifist Route.
2 Delivering Undyne's Secret Letter
At MTT Resort, there is a bridge connecting it and the CORE. Undyne calls the player on their Cell Phone once they walk there, asking them to deliver a secret letter for her. She is waiting right outside Papyrus's house, letter in hand (Undyne's Letter).
The player must deliver the letter to Alphys at the Lab. She thinks they wrote it, not Undyne. Alphys then has the player go on a date with her. It is a requirement to the True Pacifist Route they befriend Alphys.
1 Welcome To The True Lab
The player will receive a call from Papyrus following leave the Garbage Dump and befriending Alphys. He tells them to go to Alphys's Lab. There, the player finds a note; then, they go to her bathroom which is not actually a bathroom but rather an elevator that leads to the True Lab.
If the True Lab is revealed, the True Pacific Route was successfully achieved. Inside, four keys must be put into their respective slots. Once completed, the generator room is opened. After turning on the generator and talking to Alphys once more, the player takes the elevator and is warped to New Home. There is no turning back now; this where the story comes to a close. Asgore (and Flowey) await one last time.