The story of Solartale stays true to the original story of Undertale until the Monsters are locked away from humanity. To ensure that there was no chance of The Monsters returning, the humans sealed them away in a 4-Planet System 500,000 Parsecs away from earth. The Monsters then colonized the system in hopes of survival. Over time, 6 humans who were on exploring expeditions crashed onto one of the planets, labeled "Ruin" for the mass wreckage on the surface, died after leaving Ruin. Approximately 10 years after the sixth human died, The 7th human crash landed on Ruin. They left Ruin, but did not die. Shortly afterwards, all monsters were freed from the constraints of the system by the 7th human and Asriel reborn, afterwards, a peace treaty was signed on Earth and Monsters and humans ruled the Solar System in peace.
There are 5 runs in Solartale. There are the Pacifist, Neutral, Genocide, And Soulless Pacifist endings as there are in the base Undertale Game, but there are 4 extra endings.
If the player does 3 Genocide Runs, they will be permanently locked out of the Genocide Ending, but the Hardcore Genocide Runs will be unlocked. This run has a few changes:
- Sans will immediately walk in front of the player in Snowdin, Provoking the Enraged Sans fight. This fight can be skipped if the player did not kill everyone in the Ruins by using the talk command, however, if the player does a partial-Genocide run (A Neutral Ending) where they kill Flowey, they will have to fight Enraged Sans again, but with no battle menu.
- Asgore will replace Sans in the Last Corridor, but the fight will operate the same.
- Sans will be in the Throne Room, And the player will have to fight him as the final boss in an 1-Hour long fight. If the Player kills Sans after the fight, they are confronted by Chara, who says that even they don't understand why the player continuously destroys and brings back the world, Followed by them saying "You already know how this works." Followed by the Erase/Do Not in a Genocide Run, however, Chara says to the player "You should make your decision as far as this world goes. I won't let you bring this world back if you Erase it, However, I will understand if you do not choose to erase this world. But, when you are ready to delete this world, I will be waiting in this room, Also, don't try to reset this world, I'll kill you if you do." If the player then resets, The player will have to fight Chara.
- If the player spares Sans, that will lead them to the Nightmare Genocide Ending, in which the player murders Sans in his sleep, followed by Chara looking at the player and then a Chara Jumpscare.
If the Player only kills Toriel and Undyne, they will be in an Havoc Run, This is simply a Neutral run with a Different Ending. The player leaves the Underground using salvage from their ship in the Ruins, but a Cutscene will be shown after the player leaves where Sans calls the player and says this, as a cutscene showing the Outer Worlds break into civil war is played in the background.
"Heya, kiddo. I hope you've been doing good. I haven't. Things have gotten pretty bad out here. The Royal Family is dead, and, uh... We didn't have any backups, so the outer worlds broke out into war. Me and Flowey are on Ruin, which has pretty much become the only untouched planet. Alphys is trying to make a artificial Human Soul to break the barrier, and just about everyone is trying to make a giant city. Once Alphys has made that AS though, We'll start building a ship to fit all the outer world's people in, But that could take 10 years with the resources we have. Me, Flowey, and Grillby are living in a little house we built. We had to make the entire thing fireproof. You can guess why. Anyway, I'll see you on earth after the AS is done."
Insanity has said that he bore inspiration for Solartale from another Undertale AU known as Outertale, however, he does says that Solartale was not made to rival Outertale, nor is it a direct copy of it.
Insanity has said that anyone is welcome to make artwork, fan games, and even comics for Solartale.
The Leitmotifs of Solartale are very noticeably different from any other AU. Insanity said he did this for 3 reasons: 1: He's not very good with music. 2: He wanted Solartale to be easily distinguishable, and 3: He didn't want to steal music from the other Space undertale AU, Outertale.