Here’s the outline.

- For unknown reasons, Dusttale will have a passageway to the multiverse.

- Because of differences in ideology and personality, the two fight and become friends. 

- Murder!sans asks UF!sans to judge him when that time comes in case he becomes a mindless monster who has lost his reason for even stopping the human.

- By repeated reset, he gradually loses his reason, and the time for judgment comes.

- External intervention causes fatal errors in the Dusttale, and the timeline is destroyed. (In this process, UF!sans is banished to the Underfell.)

- The recovered timeline is Undetale, not Dusttale, but there will be a time when it will change back to Dusttale someday.