UNDERTALE: HARD MODE, at first sight, has an identical plot to the original Undertale: a war between humans and monsters, monsters being banished to the underground, Mount Ebott being the link between the surface and the underground, a child (who's name is chosen at the beginning of the game) falling into the underground, that child meeting many monsters, and the child choosing between befriending every monster they meet or killing every monster they meet. Like Undertale, there are multiple paths that can be chosen depending on how many monsters are killed: no monsters result in Pacifist, some monsters result in Neutral, and all monsters result in Genocide.
However, a major feature of UNDERTALE: HARD MODE is that there are more paths to take after the first time a player completes either a Pacifist or Genocide route: two more routes after completing Pacifist once, and two more routes after completing Genocide once.
By completing Pacifist once, and completing Pacifist again immediately after, the game doesn't end with Alic, Rilo, Forrest, Azura, Lanse, and Indi. There is no order that the Humans must be fought, with the exception of Indi, who must be fought last, as he refuses to fight Frisk without knowing that they are powerful enough to defeat the first five Humans. Upon having Indi spare you, however, an unseen force strikes Indi down, destroying his SOUL. This force reveals themself to be none other then the First Human, Chara. Chara came, sick and tired of watching this timeline's Frisk forgiving and not killing like she wanted: so, they stole hundreds of SOULS: Frisk's SOULS, specifically. Frisk must fight Chara as she destroys each and every SOUL to attack. During the end, Chara tires and can't bring herself to kill Frisk, as Frisk forgives Chara for controlling everyone and for all of the timelines she ruined: Chara then uses their SOUL, Frisk's SOUL, and the remaining humans' SOULS to break open the barrier, allowing the monsters to go to the surface, and ending the game's True Pacifist Route.. Rather, after the second Pacifist Run, the monsters are still in the underground: however, Dr. Alphys discovers that the Six Human Souls have scattered across the underground, and the power they gained when Flowey absorbed them allowed them to take on corporeal forms. Frisk must go back, find the Six Humans, and fight them to return them to Soul Form and break the barrier: these six humans being
By completing Genocide Run twice in a row, upon reaching the Judgement Hall, Sans will be gone: where he would usually be is a pile of dust with his blue parka, as well as a visible knife with what appears to be blood (but smells like ketchup) called Sharp Blade, one of the most powerful weapons in the game. When the player walks to the throne room, Asgore is still there, but, when Frisk/Chara attempts to attack him, a Gaster Blaster will block the attack, and Asgore hurriedly leaves. Suddenly, a spirit appears and confronts the player: none other then W.D. Gaster, revived after absorbing the six Human Souls, and angered that the player has destroyed the society he once lived in. After an extremely long boss fight against Gaster, he finally is defeated, and the six souls are destroyed. Flowey will appear to finish Gaster off, trying to convince the player that they never betrayed them, but Flowey is killed like in the original genocide route. From this point forward, the player will look like Chara in the overworld. After reaching Asgore in the barrier and killing them, the player attempts to go through the barrier. However, the ghosts of the monsters the player killed corner Chara, and the game crashes. Upon rebooting the game, Frisk appears to the player, explaining that they are what is called "pure determination", and that Chara's determination could've been used for good, rather then massacre. Frisk then traps Chara in the bullet box, fires millions of attacks at them before the SOUL is destroyed, and Frisk warns Chara, "not to make that same mistake ever again". The game then resets completely, erasing the original save file. This is True Genocide.