The Gist

Undertale is an Indie Action RPG created by Toby Fox, with art done by his friend Temmie Chang, funded via Kickstarter back in 2016.

The plot is that you are a small human child of indeterminate gender (though usually in the fandom referred to as a girl), who fell into a mountain, down into the underground world of monsters. In the history of the world, humans banished monsters below after a great war, and they put up a magical barrier to prevent monsters from getting out, though humans (with their unique human souls-- which monsters lack) are able to pass though the barrier freely. The king, Asgore, has sworn to kill any human who comes to the underground, with the stated goal being that he will gather enough human souls to break the whole barrier down. Also kicking around the place is a mysterious and malevolent yellow flower named Flowey who claims to know you and hates you for no discernible reason, and also seems to know when you reload your game.

You spend the story meeting and befriending (or killing) the assorted monsters you come across on your way out of the underground.

Notably, the game has a myriad of similar neutral endings (depending on how many monsters you killed, and who is left alive at the end of the game), the Pacifist Run (which is the canonical good path, with lots of extra content and the true ending of the game), and the Genocide Run (where you explicitly and intentionally kill every possible character, including farming random encounters in every area until there are no random encounters left). While the Pacifist Run, again, is the canonical Good Ending and has a longer, more involved plot and far more character development for all the people you encounter, the Genocide Run has a completely different (albeit much shorter) story line, and in it you find out some information about characters who were left ambiguous in the Pacifist Run.

You'll notice that nowhere in that synopsis is any mention of Gaster.

Background: Alphys

One of the main characters you meet in the game is Doctor Alphys.

She is the anxious, dinosaur-like head scientist for King Asgore who, as you'll find out as the game progresses, is obsessed with human media, specifically anime. As the only human in the underground, she becomes determined to help you along your quest, despite the fact that she works directly under King Asgore, a man who wants you dead.

Through the course of the story (if you play a Pacifist Run), you learn that Alphys has a number of hangups; she has relatable self-deprecating humor and a crush on her friend Undyne, the boisterous head of the Royal Guard, and that she's so desperate to feel at all heroic that she's begun intentionally setting up obstacles in your way, just so she could come in and save you (which winds up going terribly awry). You help her overcome her timidity, get her and Undyne to have a heart-to-heart, and eventually uncover the real reason she seems so self-loathing.

As you travel through an area of the game called the True Lab, you find out why Alphys has such a low opinion of herself: it isn't just because she's a self-depreciating millennial, it's because in the course of her work with Asgore, she's had to do a number of ethically questionable experiments on her fellow underground citizens to disastrous results. Alphys is overcome by the guilt of her failures and her inability to contact the families of those who have suffered due to her experiments. In the Pacifist Run/Canonical Good Ending of the game, you help Alphys confront the consequences of her actions, and she reaches out to the families of the test subjects. In more than one bad ending of the game (specifically the ones where you kill Undyne or Alphys' robot friend, Mettaton), it is heavily implied that Alphys kills herself due to both the loss of whatever character you've murdered as well as the guilt she feels about the experiments.

Background: Sans

Sans is another character you meet, though at first glance he doesn't seem to have the depth Alphys has.

He is a skeleton, one of two skeleton brothers who are new to the underground monster world, and he's the first monster you meet once you leave the tutorial section or "The Ruins." He is a prankster and layabout with a laissez faire attitude, and whose name and personality are a reference to his speaking font, Comic Sans. His emphatic, comedically egotistical, and incompetent brother, Papyrus (whose text is in Papyrus font) criticizes him for being lazy, and Sans rolls with it. He isn't interested in capturing you for King Asgore's sake, instead he asks if you'll help him make his brother happy by going along with Papyrus's overwrought human traps (which consist mostly of harmless puzzles). For the rest of the game, Sans appears around the place (due in part to his heavily implied, but never expressly stated, ability to teleport), usually offering levity through humorous interactions, such as when he sells you hot dogs and stacks them on your character's head, or when he tricks you into looking into a telescope that has ink on the rim, staining your character's face.

There are only two times in a Pacifist Run where we see that not all is as it seems with Sans:

At one point, he'll invite you to have a meal with him and he explains how he was sent to guard the area outside the Ruins where you spent the tutorial. He says how he heard a woman through the sealed door to the ruins, and how the two of them became friends by telling each other jokes. He recounts how the woman one day asked him not to harm any humans who came from the ruins. Then he tells you, with his eyes suddenly turning completely black, his text-speak bold and slow, that if not for her request, he would have killed you the moment he saw you.

The woman, players will know, is Toriel, the goat-woman who cared for you at the beginning of the game, and who had in the past seen several other humans fall into her ruins and leave her, only to be slaughtered by Asgore and his guards.

After that reveal, Sans casually changes the subject.

The other incident that reveals Sans may have some hidden, and not entirely pleasant, depths are in the "True Corridor", colloquially known as the Hall of Judgement. As the player is about to confront King Asgore, they find Sans standing in a golden hallway, and in every play through except for a true Genocide Run, he comments on your actions and lets you pass. In a Pacifist Run, he comments on the player's compassion. In some neutral runs with a low death count, he says maybe you did what you thought you needed to do, or at least, you weren't as bad as you could have been. If you killed his brother, he is clearly pissed, but it is a subdued and unnerving rage that isn't dwelled on.

However, that all changes in a Genocide Run.

In the Genocide Run, Sans is the most difficult boss battle of the entire game --which the game mocks you for; Sans has only 1 hp, but it is impossible to actually hit him. The fight is designed so that he will continuously dodge your attacks, and you have to survive his attacks long enough to run through all of his dialogue. If you die and come back to fight, he knows. He'll comment on how frustrated you look that you have to fight him again and again, then proceed to blast you with his strange, dragon-headed weapons that the image files for the game call Gaster Blasters

He will say things about how "our reports showed a massive anomaly in the timespace continuum" and there are "timelines jumping left and right, stopping and starting. . .Until suddenly, everything ends.Heh heh heh. . .That's your fault, isn't it?" And how he had been hoping that whatever anomaly was causing all the timelines to restart and reload would chill out if it had some friends to ground it, but clearly (given that you've just murdered almost everyone that it is possible to murder) he was wrong.

In Neutral/Pacifist runs of the game, characters from Snowdin town will tell you that Sans and Papyrus just showed up one day and made themselves part of the community. In Sans' battle, he lets slip that he 'gave up trying to go back a long time ago. And getting to the surface doesn't really appeal anymore, either," confirming that he and Papyrus really are from somewhere entirely different than the surface world or the Underground-- which ought to be the only two options.

Also, in a Pacifist/Neutral run, you can visit the house Sans and Papyrus share, and will find Sans' room locked. If you do a series of annoying and time consuming tasks that requires you to beat a Neutral run and then restart, Sans will give you a key to get into his room. Once there, aside from some funny flavor text, (as well as a throwaway line from papyrus about Sans' pranking people from across time and space), you'll find another key that opens up Sans' Workshop-- a secret location hidden behind the house and located in the basement. Inside, there's a strange, enormous, and hopelessly broken machine covered by tarps, blueprints for machines the player character doesn't understand, and a drawer with photographs of people your character doesn't know, as well as a badge. In a later update to the game, after interacting with a rare, random event NPC, another picture is found in one of the drawers; a poorly drawn picture of three people with the words "don't forget" written underneath.

Though none of these elements are ever extensively brought up in game (with the most blatant references being the Genocide Run battle dialogue), with the pieces we have, it can be reasonably assumed that Sans and Papyrus are from some other dimension/world (something that is nearly confirmed in the sequel game, Deltarune), that Sans is some kind of time cop/scientist who is aware on some level of the player restarting/loading the game, that the brothers are now stuck in this world, and that the broken machine in the basement is likely some kind of time/dimensional travel device and is probably responsible for the two being stranded.

. . . So What About W.D. Gaster?

W.D. Gaster was the royal scientist before Alphys took over, and likely the third skeleton brother (if not their father or uncle).

He also doesn't exist.

I do not mean that he was a character who was dropped from the game and dummied out, but that he is a character specifically designed to no longer exist. For much of the game's history the only way to find out anything about him was through messing around with the game files, most noticeably the "fun" value of certain locations-- fun values being a randomly generated number between 1 and 100 that was applied to locations in the game every time you saved/loaded a file. The numbers were in charge of random events in the game (for instance, determining which character would give you a phone call at a certain location), but if you messed with a location's fun file, (usually by capitalizing the word fun and editing the number as necessary) it would result in rooms and characters appearing who do not appear in the normal game at all.

In version 100.1 of the game, after all the updates and Switch ports and the release of next game Deltarune, these encounters are now in the game proper, though still available only through rare random events.

Gaster Followers

Of the five visible characters you could spawn via the fun files, three of them are Gaster Followers, named for the name of their sprites ("spr_g_follower_1" "spr_g_follower_2" and "spr_g_follower_3"). These are the only characters in the game who mention Gaster specifically. These characters appear in Hotland (the area before the True Lab mentioned in the Alphys section above).

Unlike other character sprites, which are monochrome in-battle but are colorful during normal gameplay, the Gaster Followers are all grayscale.

They are the only characters who talk to you about W.D. Gaster.

The first Gaster Follower tells you:

It makes sense why ASGORE took so long to hire a new Royal Scientist.
After all, the old one... Doctor Gaster. What an act to follow!
They say he created the CORE.
However, his life... was cut short.
One day, he fell into his creation, and...
Will Alphys end up the same way?

The second says:

Alphys might work faster. But the old Royal Scientist, Doctor W.D. Gaster?
One day, he vanished without a trace.
They say he shattered across time and space.
Ha Ha... how can I say so without fear?
I'm holding a piece of him right here.

And the third says:

I understand why ASGORE waited so long to hire a new royal scientist.
The previous one... Dr. Gaster.
His brilliance was irreplaceable.
However, his life... was cut short.
One day, his experiments went wrong, and...
Well, I needn't gossip.
After all, it's rude to talk about someone who's listening.

Entry Seventeen and Gaster's Theme

I mentioned earlier that part of Alphys' story includes you noodling around the True Lab, uncovering the dark secrets she's been keeping about her experiments. The way you do this is by finding log entries detailing the whole sordid affair and then confronting her.

If the fun levels are right, then you can access a room in the True Lab, room number 264 in the game files and called "room_gaster." However, instead of it being an actual area, it is instead a black screen with shaky white wingdings font giving you a 17th log entry that is not written by Alphys.

It reads:

ENTRY NUMBER SEVENTEEN
DARK
DARKER
YET DARKER
THE DARKNESS KEEPS GROWING
THE SHADOWS CUTTING DEEPER
PHOTON READINGS NEGATIVE
THIS NEXT EXPERIMENT
SEEMS
VERY
VERY
INTERESTING
...
WHAT DO YOU TWO THINK

After which, the game will close.

Another fun value specific location is the Sound Test Room, a hidden room that has a chance of showing up if the values are at 65. In this room, you can listen to several songs: Happy Town, Meat Factory, Trouble Jingle, and Gaster's Theme. If the player selects Gaster's Theme, the song will play, and then character will be booted out of the room. After the update, the room also gives a message of "Thanks for your feedback! Be seeing you soon!" when it boots you.

The Man Himself

So if you messed around with the fun values back in the day, or if you get very lucky now, sometimes a hallway between two areas will mysteriously have an extra section of hallway with a mysterious door. This door leads to Mystery Man, so named for his sprites. He has a big white head with some kind of crack in it going down to his eye, and a black outfit that looks like a face when upside down (which is a common aesthetic choice in Undertale sprites is that a lot of monster designs have optical illusions). He also has no collision physics, and if you walk into him, he looks surprised, then vanishes. If you leave the room and hallway and try to come back, you'll find the hallway has gone back to normal and the room isn't there.,

People have latched onto this design, despite the fact that technically that character isn't actually referred to as Gaster. This is because in the game's files, there's unused battle data for a nonexistent W.D.Gaster fight. His stats are all 6s. The fun value to get the Mystery Man room to show up is 66, hence people making the connection between the two.

If you notice that a lot of fanart seems to center around Gaster having a bunch of creepy, floaty hands, this is because of a line spoken by River Person. In the game, there's a fast travel option through a character named "River Person" who takes you on a boat ride to other areas. While you travel, River person will say some line of dialog. Usually it's a random joke, or some kind of gameplay hint, but once in a while, they'll say "beware the man who speaks in hands".

Given that Gaster's font is a version of Wingdings with hands used in the text, people assume that he is to whom River Person is referring, and went hog wild on the aesthetic.

So to recap:

We know Gaster is a scientist who was working on something big that got him shattered across time and space, and removed his presence from the actual game (at least until the update).

We can assume he is related to Sans and Papyrus because, like Sans and Papyrus, his name is based on the font(s) he writes in (Wingdings and Aster). We can assume a greater connection between him and Sans, since both have a weird time/dimensional science connection (Sans being some kind of timeline-monitoring, stranded space/time cop/scientist). Toby Fox commented on people's messing with the fun values on twitter and mentioned that "neither" of "them could fix the machine. Presumably, this is the machine in Sans' workshop, though it may be whatever experiment Gaster was working on when he got himself retconned out of existence.

In his Entry Seventeen log, he asks what "you two" think. This could be referring to Sans and Papyrus, but could also be Sans and Alphys.

And after that. . . nothing.

So What?

W.D. Gaster gave Undertale fans a mystery that not a lot of games-- especially one-man indie games-- have provided. When the game was released, you could play all of Undertale, experience all three ending types and even do all of the dozen or so neutral endings, and never once encounter anything Gatser related. Hell, even now the random events that conjure up the Gaster followers are still pretty rare, so you could still play Undertale and never hear about the man.

People have been tearing into game files to find goodies since forever, but to go in and find a plot, with all sorts of little breadcrumbs around for you to find and put together, especially back in 2016, was unheard of for most players. These days, the practice is becoming more popular, with games like Doki Doki Literature Club requiring the player to go into the game files and literally delete the villain, but back then it was pretty mind blowing for a lot of people.

The Future: Deltarune???

Now, a brief aside about Deltarune.

Deltarune is an as-of-yet incomplete game made by Toby Fox. The first chapter is released, and it is in and of itself a complete (but short) story. It takes place in an Alternate Universe version of Undertale, where monsters are on the surface, and for no stated reason, all the humans except your character, Kris, are gone. The characters in Deltarune are the same as the ones from Undertale, but with different histories, meaning they aren't the same people; characters who were friends now hate each other, characters who were mean to you are nice to you, etc.

According to Toby Fox, he actually had the idea of Deltarune long, long before Undertale, and Undertale came about while he was planning the story for Deltarune. He essentially paused working on Deltarune (which he said was rapidly becoming a very, very large project), and focused on Undertale instead.

So imagine how shocked the entire fandom was when, after the release of the first chapter of Deltarune, they all realized that there had been a website called Deltarune.com from 2015, with an entirely black screen and a hidden message in wingdings, which was an excerpt of Entry Seventeen. That, coupled with the Undertale update's "Thanks for your feedback! Be seeing you soon!" in the Sound Test Room and a few other minor Deltarune-related things have led many to assume that if we are ever going to get the full story behind W.D.Gaster, it's going to be later on in Deltarune.

Grandpa Semi and Weird Metanarrative Assonance

The Korean demo for Undertale way back when had a lot of the original content inside that was cut from the English versions. I don't mean actual gameplay or plot or anything a casual player would notice, but there were a few more junky, dummied out files inside the game itself from the original version. A lot of these files were only accessible through datamining, and one such file was an unused audiofile called "grandpasemi.ogg"

The audio itself is very similar to the first few seconds of "Metal Crusher," a song that plays when you fight Alphys' robot friend. Dataminers also found that Grandpa Semi, whoever he was, was going to give the player a phone call. In the current game, that phone call is replaced by Alphys mistakenly calling you instead.

Finally, Toby Fox released photos of his old college notebook where he had written down ideas for Undertale, and users on Reddit edited the contrast in order to try and see what the pages behind the ones being displayed said. In them they found a mention of grandpa Semi's name in a section talking about the skeleton brothers, Papyrus and Sans. Because Sans and Papyrus have names relevant to fonts, the popular theory with Semi is that he would have been their grandfather, and that his name was a reference to Semi Serif fonts.

However, clearly this didn't pan out. There is no Grandpa Semi in the game. A later update changed the name of the Grandpa Semi audio to "Grandpa Temi" and was used in a special encounter with a monster named Temmie found in the Temmie Village (which is a special/joke area filled with monsters who are all named Temmie, except one named Bob).

Where the Gaster connection comes in is that fans speculate that the removal of Semi is what may have inspired the creation of W.D. Gaster to begin with. Semi was likely a skeleton relative of the brothers who was ultimately removed from the existence of the game outside of the narrative. Gaster is likely a skeleton relative of the brothers who was removed from the existence of the game within the narrative.

Does this change anything? No, but it is a neat bit of digital carbon dating that lets us see, potentially, how one creative decision inspired another.