Undertale is the story of a human child who falls into the underground, which has become the home and prison of the monster race ever since humans drove them from the surface. Monsters have set up a life in this new place, but long to return to the surface.
The Neutral Route is one of the three primary routes in Undertale. In this route, the final boss is Photoshop Flowey, though after fighting him once (without performing a True Reset or a Genocide Route), Flowey only destroys Asgore's SOUL and his fight is skipped, as he knows the SOULs would revolt again.
On a True Pacifist Route, the protagonist must reach one of the Neutral Endings to unlock the quest line leading to the True Pacifist Ending.
This ending occurs when the requirements for either the True Pacifist Route or a Genocide Route have not been met, or if the protagonist on their first playthrough did not kill anyone as a prerequisite to the True Pacifist Route.
There are many versions of this ending, all consisting of Sans calling the protagonist's Cell Phone and leaving a message telling the protagonist of the Underground's fate. However, the content of the phone call is based upon the actions that the protagonist has made throughout the game. The track An Ending plays during the call, except in the Mettaton, Leaderless, Dog, Alphys, and Impossible variants.
In all of the Neutral Route endings (except for after aborting the Genocide Route in Hotland), Alphys tells the protagonist that they must kill Asgore and take his SOUL, as a human SOUL alone cannot pass through the barrier. Consistent across all Neutral Route epilogues:
After the brief, lackluster credits of the Neutral Route, Sans calls the protagonist's Cell Phone to leave a message. He can begin the call with a special comment if certain requirements are met. Sans will only ever make one comment; this table lists them in order of descending precedence.
(For example, if the protagonist neither removed the Bandage nor used consumables, and did not kill Papyrus, Sans will give his comment about getting something to eat.)
The True Pacifist Route is one of the endings of Undertale. This can be considered the "true ending," as it leads to a happy conclusion and the complete credits.
If the protagonist spares Flowey, he appears after the Neutral Route phone call. He provides a hint on how to earn the True Pacifist Ending, excluding an aborted Genocide Route attempt. If Flowey was killed, the game immediately returns to the Start Menu.
The protagonist can then reload their SAVE file to fulfill any missing requirements as long as they have not gained any EXP/LOVE. Otherwise, they must reset their game. Note that killing Asgore or Flowey does not acquire any EXP.
The list of requirements is as follows:
The Genocide Route leads to one of the three main endings of Undertale. It consists of the protagonist killing every possible monster in the Underground.
This route's ending can occur without prior completion of the game, unlike the True Pacifist. Once the protagonist completes the Genocide Route, subsequent True Pacifist Route and Genocide Route endings are altered – even after a True Reset, being informally called "soulless routes".
The Genocide Route occurs when the protagonist kills every enemy (including mini, mid and main bosses) in every region: the Ruins, Snowdin Forest, Waterfall, and Hotland (the CORE is a sub-region of Hotland).
Starting in the Ruins, the protagonist repeatedly triggers encounters to kill all monsters. This is similar to level-grinding, which is earning experience points before progressing in other RPGs. After killing all monsters in an area, encounters still occur, but monsters do not appear.
In a font size smaller than usual, the battle text reads "But nobody came.", which can also occur in a Neutral Route after killing all monsters in an area. This also changes the SAVE Point text to only state "Determination." instead of the usual lines.
After the small battle text message appears, the music for the area changes to a distorted ambient track, mus_toomuch.ogg. This track is a part of Your Best Friend playing at 1/13th speed (~0.077x or ~92% slower) with added reverb.
After exiting the Ruins, most overworld songs are lower pitch, slower versions of their Neutral and True Pacifist Route counterparts. Some NPCs no longer appear, and slight alterations occur in some locations, such as most puzzles already being solved.
If the protagonist stops meeting the requirements for a Genocide Route at any point, they revert to a Neutral Route where the unaltered soundtrack plays. NPCs still occur in the region where the protagonist aborted the Genocide Route.
Kill counters for different areas have different requirements. If certain events are triggered before clearing an area completely of monsters, the protagonist is automatically sent back to the Neutral Route. For example, if Papyrus's battle is initiated
without exhausting the Snowdin kill counter, the cutscene proceeds as if on the Neutral Route. Even if the protagonist is captured and then goes back to finish off the last monsters, re-fighting Papyrus still proceeds as on the Neutral Route. Once a Genocide Route is aborted, it cannot be returned to without reloading to a SAVE before it was aborted or resetting entirely.
There is a glitch if the protagonist enters the fight with Toriel. If there is still one possible encounter left in the Ruins, the fight with Toriel proceeds as normal for a Neutral route; however, if Toriel is killed, the protagonist continues a Genocide Route.