Board Thread:General Discussion/@comment-26006155-20190617222636/@comment-26006155-20190621132325

No.. I said that Sans and Gaster warped into the Underground, used their Anime Tech gear (they came in a time machine, after all) to spy on the locals, then devised disguises based on the intel they recovered from those observations.

It's important to note that Undertale itself is not the start of the story.

When Frisk looks at Asgore's calendar with that date Chara arrived circled on it (The end of 201X. So, around Christmas 2019), the narrator notes this is an old calendar.

In the Beta Timeline, Beta Gaster died, and had a memorial fountain built for him, in 201X, before Chara even arrived. Which means that the arrival of Sans and Gaster in the Underground, Gaster's rise to Royal Scientist, the construction of the Core, and all of Gaster's research later recovered by Alphys, all happened before Chara even arrived.

Then we have the Chara and Asriel story, where their Doomed Buttercup Adventure happened, at the very earliest, in Spring 2020 when the flowers bloomed again.

THEN we have the Six Humans who followed Chara.

And finally.. Frisk begins his story in the actualy game.

There's a lot of history that happens before Undertale even begins.

At some point, somehow, Sans ended up in the ruins without his memory. Then later recovered it. And new that his earlier self would retrace these steps, which is why he and Papyrus took up guard duty outside the ruins.