The XTALE comic takes place in Timeline X, where everything seems to be going normally, until X-Tale!Sans starts having some memories of a supposed genocide run, a direct fault of a secret mastermind. This would lead to a series of events that would finally bring an end to the universe.
For more information about timeline ten, see XTALE X.
Years after XTale!Frisk shattered the barrier, X-Tale!Sans and X-Tale!Papyrus are taken by surprise by XTale!Undyne, who ends up kidnapping who they were tasked to protect, XTale!Frisk. This turned out to be a test, however in a freak moment, XSans almost kills XTale!Frisk in the process. With the two returning XFrisk to his family, XSans, XPapyrus, and XGaster return home yet XSans is slightly concerned over the incident that he experienced. XGaster takes note of this change, examining if he should overwrite or not (something he's unable to do at the time). Meanwhile, Sans begins dreaming of X-Event!Chara, who threatens to meet again if he doesn't remember what he's tasked to do: kill XFrisk.
The next day, while at a ceremony to promote peace between humans and monsters, XSans tries again to kill XTale!Frisk, which results in XGaster intervened. He flees, only to be caught by XTale!Mettaton, and before he could explain to his brother on his current situation, the timeline resets again, this time with the ceremony going on normally.
As it turns out, Cross figures out that X-Event!Frisk (and by extension X-Event!Chara) and XGaster are both fighting against each other for control of the timeline. X-Event!Frisk tells him the truth about the situation, and how this universe was rewritten ten times thanks to the power of overwriting. Despite X-Event!Frisk's plead for XSans to kill him, XSans' refusal leads to a battle which ends up getting XSans his signature scar, and the name "Cross". In the aftermath of the skirmish, XGaster resets the timeline once more, but his actions garnered the attention of X-Event!Chara, who realizes that they have the only being in their universe to overwrite.
Realizing that he's back to when XUndyne does the test for him and XPapyrus, Cross is somewhat baffled by the events that had happened before the previous reset. As both him and XPapyrus head out to town hall with X-Event!Frisk, XUndyne and XPapyrus both betray him, on the onset that both have their memory restored of the past nine timelines and realizing that Cross is refusing to cooperate with the X-Event to kill XGaster. Cross survives however, and he plots his revenge.
In the meantime, XGaster is being taken out by the rest of XTALE cast minus XAlphys. The X-Event demands to know where XGaster hid his overwrite soul, to where he warns the X-Event that his other half is hidden within their grasp and that he'll oversee their struggle to obtain the missing part of his soul. Just in the middle of the confrontation however, Cross surprises the cast, kicks out XGaster, and proceeds to murder everyone including his brother so that he could become the new X-Event. Cross succeeds in his mission, but he soon finds out that due to limitations that XGaster had set on the X-Event soul, he could not overwrite within his grasp. Cross is visibly upset about this, but X-Event!Chara, whom is now part of him, is easily able to sway him on his side, just as Ink!Sans appears in the universe.
The main protagonist, X-Tale!Sans/Cross is unassured after he begins having problems related to his code (e.g. He wants to kill X-Tale!Frisk). His actions have placed him as a tragic hero, is his own fault for his universe's failure to exist.
X-Event!Frisk is the main antagonist of the series, along with X-Event!Chara. The two of them keep messing around with X-Tale!Sans's own coding to try to kill themselves (possibly to end the life of the X-Event itself permanantly, which would also prevent XGaster from overwriting the timeline endlessly), and once that fails, he begins rallying up support against XGaster with the rest of the cast. He fails to, however, kill Cross, and this results in him being killed after Cross takes their soul, during a heated confronted between his brother and Cross.
X-Event!Chara is X-Event!Frisk's other half of the X-Event pair, expressing his interest in disposing XGaster as well. He plays no specific role until the near end of the comic, at which he expresses his disbelief that his brother is gone, and repeatedly attack Cross for what he did.
XGaster is the creator of X-Tale and the tritagonist of the series. Though he's equipped with his overwrite ability, he's unable to use it thanks to his soul's lack of power, allowing his kids to use it at will. It is revealed soon afterwards that he hid the soul somewhere else to prevent them from taking it, and after he's ejected by Cross to fight the cast, he's completely wiped from his universe, expressing interest to see what his X-Event can do.