About the AU
The plot of Underchaser revolves around Chaser!Sans as he moves through other worlds in hopes to figure what happened to his world.
Worlds end arc
Frisk jumps off to end their life but fortunately survives the fall, deciding to keep moving forward meets with a strange skeleton who promptly kills Frisk, having some unknown power Frisk keeps coming back making this skeleton remember what he had done after a few times. Noticing he had mistaken them for Chara, he ties them up in ropes to get them to the president.
Meeting Toriel half-way they present each other and she offers a place to stay thinking Sans (the skeleton) was hurt due to his nose starting bleeding. Sans and Frisk rest up in a "chills" room while they comment about their pasts.
Frisk had been teased and bullied and saw no other option but to try to kill themself, while Sans on the other end, saw everyone from his world die and after a hard battle with a "demon", he saw himself alone in his world and using the soul he captured he erased his world after listening to some voice advising him to do so. After that event, his world returned to a normal state but he had been forgotten by everyone else. He took it upon himself to find the human before the same event happened.
After ending up telling his past the world suddenly turned white and then Sans ended up in another world. Waking up to a violent welcome he met another skeleton, to distinguish he told Sans to call him Red since both oddly enough shared the same name. After trying to explain the events that just happened and not being taken seriously he started to work to Papyrus of that world.
After some weeks some monsters start disappearing and a strange creature attacks Sans and Red as they were doing rounds to verify what's happening. Sans takes on the opponent while Red calls Papyrus and after a small battle, the creature vanishes. Leaving Sans a little worried he promises to take care of Red and his brother Papyrus.
The next day after their rounds Sans and Red starting discussing near Papyrus, Papyrus loses his color and then vanishes leaving both Sans and Red in a panic, Sans asks Red to give him his hand and both end up vanishing to some dark void.
Sans is able to keep his conscience and has a chat with a human from his realm that admits they made a mistake. Sans is confused about it but his time is up and he wakes up with Red in a jail. A little skeleton presents himself to them.
Having told what happened to the new skeleton the 3 formed a search team in order to find any clues of what's causing the worlds to vanish.
Feeling the drawing sphere falling appart the AU protector Ink, went to question the AU destroyer error about the destruction of many universes. Error pointing Ink on Chaser!Sans direction putting him the blame for the destruction fo the worlds.
Meanwhile the trio started searching, convincing the head of the royal guard to get their troops searching for possible menaces, the underground was united to stop this menace. Swap!Papyrus and Red found another weird creature after knowing that Muffet was gone and beat it with teamwork. Meanwhile Ink had arrived to face Chaser!Sans, to his surprise he was asleep he thought it would be an easy kill, but as he attacked a shield deflected his attack, deciding to convince Chaser!Sans that it was only a matter of time until the rest of the universes dissapeared, Chaser!Sans decided that he should destroy himself in order to the leftover worlds to keep existing.