|“||I don't want to regret it now.||„|
|~ Ink!Sans's reply towards Classic!Sans about his feelings|
Underverse 0.4 is the seventh episode of Underverse, and the final episode of Season 1. Without counting Underverse 0.3 Part 1 & Part 2 as one episode, it is by far the longest singular episode in the series.
XGaster prepares for his revival as X-Tale!Alphys quarantines the corrupted timelines. In the meantime, Nightmare!Sans gets into a frenzy with X-Event!Chara, Sans goes to find Ink!Sans, and Error!Sans is ready to face off with Ink.
Somewhere in the Doodle Sphere, XGaster desires to take over the X-Event's plan of one universe but with his own ideas. X-Tale!Alphys is sent out to the corrupted timelines the X-Event caused, first with Underswap, as Core!Frisk notices her after Underswap!Papyrus & Underswap!Chara die, and then Underfell, in which Alphys is forced to summon X-Tale!Mettaton to kill the inhabitants that desired to kill the intruders; Core returns to X-Tale to warn Dream!Sans of the inevitable.
X-Event!Chara, still recovering from the events of Underverse 0.3, is suddenly scolded and tackled by Nightmare!Sans, who knows everything about him and his AU. Demanding that they head to where Ink!Sans is to get the vial, Nightmare declares he's his property and his grasp on Chara grows stronger enough to kill him. He's dropped as the objects he took are removed; Nightmare delivers Chara his partner Killer!Sans, who gives him a lesson about alternate timelines being that he's in one himself.
As the 3 Sanses talked about what just happened, Sans vows to find Ink. As soon as he enters the city, Outertale!Sans finds him and they both head out to find Ink. As Outer tells Classic his life here, he mistakes Killer as Ink, and peeking though, Nightmare attempts to force Chara to utilize monster soul. Error!Sans arrives, demanding that he knows Ink's location, to which Nightmare redirects that to Classic and Outer. Outer thrusts Classic to run as he attempts to stop the situation before fighting Killer, and as soon as Classic arrives back to warn the others, Error surprises them and takes them hostage.
The damage Error caused attracts Ink, who denies that his anomaly friends are his, and that his true friend has given him feelings much better than the other colorful vials he has, causing Classic to realize what XChara meant. Ink's ramble is a distraction enough for Error to take his vial away from him. This ends up with a 3 way battle for the vial: Error, Ink, and Chara. Nothing could stop Ink from dropping the vial though, and XGaster is reborn with half of his soul and his dying determination.
Error's response to this is to decline XGaster's offer and destroys the multiverse, to which Ink traps him in his realm, and XGaster takes the X-Event's soul and grabs the other half of his soul from their own soul, as X-Event!Chara dies without a soul to grab. XGaster knew this would happened and offered him another chance with his soul now full once more, while also killing Underfell!Sans & Underswap!Sans, sending Classic!Sans back to where he belonged, and bringing back Cross and brings his offer towards him to join him with the others before leaving him behind.
Dream jumps in to stop his brother but is caught under Killer!Sans as Cross looks around to see him close to death. He impales Killer to stop him as he and Dream escape to the Omega Timeline, where Core!Frisk is waiting.
- There was a deleted scene in the episode involving Classic!Sans and Outertale!Sans making puns, but it was removed due to it being forced as well as the fact that there was already one funny scene with them.
- During the scene where the inhabitants of Outertale are reacting to the loud whoopie cushions, there is a not-so-well hidden message that says "ATTENTION HINT TRAVELLERS: This is a spoiler. The multiverse will die today"