From Glitchtale Wiki
My Promise is the 5th episode in Season 2 of Glitchtale, and was released on April 6, 2018.
After this, Betty tosses a spear at Akumu, which impales and absorbs it. She then impales herself in the chest, thus becoming one with Akumu. She then turns into her true form, which results in her entire eye becoming pink, with her sclera, irises, and pupils becoming a lighter shade of pink. She then asks Asriel, who has been reformed using HATE, if they're ready to hunt down those who hurt him the most.
Meanwhile, Jessica confides to Undyne that she feels she has made nothing but mistakes in her life. Undyne then confesses that she feels responsible for Alphys' demise, not Betty, since she was the one to pierce Alphys' soul directly.
However, this calming moment quickly comes to an end upon them hearing rather disturbing noises. Jessica investigates and discovers that pink blobs managed to break the windows and get into the house.
Asgore and the others have arrived, with Sans still in his comatose state. Gaster heads down to his lab in the underground with his sons. Ronan then calls Jessica's phone, and Asgore answers before telling him about Jessica's situation. He asks Ronan to help regroup everyone to a safe location. Ronan decides to regroup at the school, a decision which Toriel objects to. However, Asgore tells Toriel that she resigned her seat as Queen of the Monsters when she fled Asgore, and therefore, does not have any authority in this situation.
In the lab, Gaster tries to remember his password. Suddenly, the lab triggers him into remembering flashbacks of his time in The Void. Papyrus worries about Gaster and his PTSD, but they press on to the laboratory. Gaster has Papyrus lay Sans down in a special ICU-like bed and then asks him to get him some supplies. While Papyrus is busy, Gaster views Sans' condition on the screen. It says Sans is fallen down, which is essentially a terminal condition where he will turn into dust soon. Gaster is horrified at the results as he stares at the screen in disbelief. When Papyrus gets back, Gaster hastily shuts off the computer so that he won't see his brother's diagnosis. When Papyrus asks about Sans' condition, Gaster lies to Papyrus, saying that Sans’ condition is stable and he'll recover. Papyrus asks to help Gaster with anything multiple times, but Gaster keeps saying no, to the point of summoning his Chroma hands in anger, before realizing what he did and calming down.
Frisk, Asgore, Toriel, and Undyne meet in the living room to talk about Betty. They ask Frisk why he didn't tell them about going to confront Betty. Frisk apologizes, saying that he thought he could do it on his own and didn't want to bother other people. The group accepts his apology. Asgore then explains that Betty is, in fact, a spell whose only purpose is to cause chaos. Gaster enters the room, interrupting Frisk, who was talking about how it's his fault Asriel was captured.
When Asgore attempts to get an update on Sans' condition, Gaster is unwilling to tell him and instead tells him to worry about his own matters. They discuss defeating Betty and how to free Asriel from HATE. After a brief conversation, Asgore creates a plan to defend the school. Gaster says Toriel and Asgore must save Asriel, who is corrupted by HATE. Asgore brings up that possibility of Undyne saving Alphys, under the impression that she's able to be saved the way Asriel can be.
The city suffers an explosion, which draws the attention of the crew, and they set out. A little girl is injured by one of the pink creatures, but it's destroyed by Ronan, who then tells the her to get to the school.
Betty sits on a building, seeing Frisk, Toriel, and Asgore coming. As a result, Betty comes down, and says she was excited to fight all of them, but mostly only wanted to fight Frisk and finish what they started.
Having read about the "fallen down" stage Sans is in, Gaster attempts to look for a cure. The three fighters show up, and when Gaster sees Toriel healing Asgore's wound, he becomes concerned. However, Asgore dismisses it, saying it will be fine. Both Toriel and Asgore worry if they will lose Asriel again.
Soon after, Frisk and Gaster talk about Gaster's failed experiment. Frisk says the experiment failed due to a lack of Determination and that if he recreates what Gaster did, but using his own soul, he would succeed. Gaster is surprised at first, but understands why Frisk would want to do it, which is to fix his condition.
He tells Chara this will only mean that they'll switch the places they were in back in the Underground, implying that Frisk will follow him around instead of the reverse. Chara chides Frisk for this being the only solution he can come up with, and for him being able to only last a month without a catastrophe occurring. Despite this, Chara agrees to the solution. Frisk then passes the Determination soul to Chara, pushing the experiment to the point of no return.
Frisk confesses that he lied to Chara about the "ghost following" in the Underground, and what will really happen is that he will be erased from existence. Frisk is glitched, so if he remains in the timeline, the timeline will become more unstable and erase itself eventually. Chara will lead a normal life, devoid of any memory of Frisk. Chara frantically protests, but Frisk is replaced anyway. This changes everyone's memories, while still keeping the events that actually happened the same. Chara is sent to the surface in Frisk's place. This ultimately results in Frisk being erased from existence. Frisk's final words to Chara are asking him to keep his promise.
- Despite the apparent replacement performed by Frisk, which had removed him from reality and inserted Chara, the timeline of events had not been altered. Instead, it was the memories of each individual that had interacted with Frisk up until his disappearance, that had been altered. Where Frisk had been, Chara is instead remembered to have been, and to varying degrees, Frisk's actions are credited to Chara. This appears to not include certain events that would present Chara negatively to other people, such as the argument had between Frisk and Asriel.
- The text following the after-credits scene are the exact words Goner Kid says in Undertale.