So, I was hanging with my friend the other day, and he asked me what Undertale was. I explained it to him very roughly, and I thought to myself, "golly! I could write a book! Or at least, a really long Wattpad post." So here we are. This is just a very broad explanation, but it's good enough for people who just want to know what it is, and who aren't Undertale FANATICS like I am.
Overview
Undertale was created in 2015 by game developer, nay, GENIUS Toby Fox. In addition to coming up with the storyline and a lot of the characters, Fox also wrote the entire (brilliant) soundtrack. Top tracks include Megalovania, Home, and Can You Really Call This A Hotel I Didn't Receive A Mint On My Pillow.
Now, onto the story:
Long ago, humans and monsters lived peacefully on Earth. One day, the humans threw all the monsters underground and trapped them there with a magic barrier. Anyone can enter the Underground, but only a human with a powerful soul can leave. You need seven human souls to destroy the barrier completely. The reason why you need human souls is because human souls live on after the human has died, whereas monster souls die quickly after the monster dies.
After some serious research, the research lab (AKA an adorkable dinosaur named Alphys) found the chemical that enables human souls to live on. Alphys called it DETERMINATION. She experimented with it for a while and injected it into some flowers. Not much happened.
The Pacifist Route
In the Pacifist route, the player is playing as a child named Frisk. For the intents and purposes of this post, I will call Frisk a female, as I am a female and Fox made her genderless to make her relatable to everyone.
Frisk falls into the Underground and is greeted by a friendly golden flower creatively named Flowey. Flowey actually turns out to be a psychopath and nearly kills Frisk, but Frisk is rescued by a lovable goat lady named Toriel.
Basically, Frisk befriends everyone including, but not limited to Sans, Sans's brother Papyrus, a fish warrior named Undyne, her love interest Alphys, and fabulous gay robot named Metatton. The point of the Pacifist route is to kill or harm ABSOLUTELY NO ONE. Frisk goes through the Underground until the end, where she sees the king of the Underground. His name is Asgore. He leads you to the barrier, and you need to fight him to get out. He lets you pass and the screen goes black. Frisk then gets a phone call from her friends Underground. After the phone call, she is told that she can go back to get a better ending.
Remember Flowey? He's the baddie in the Pacifist route. He is actually the son of Asgore and his wife (Toriel. I know, right?) named Asriel Dreemurr. He died and was put into the flower. However, as a flower, he had no soul and could not feel anything and eventually went insane.
Anyway, Frisk has an epic boss fight with him before Asriel absorbs all the souls and feels again. He sacrifices himself to break the barrier and everyone makes it to the surface. The End.
The Genocide Route
The Genocide Route is exactly what it sounds like. You have to kill everyone. Not just the monsters you interact with, but EVERYONE. You have to literally comb through the Underground to hunt everyone down.
It's Frisk again! After the same run-in with Flowey and being rescued by Toriel and blah blah blah, you go out and kill everyone in sight (and even not in sight). As you kill people, you level up in LOVE and EXP, but more on that later.
At the end, after everything, you are faced with Sans. At the beginning, you killed Papyrus, Sans's brother, and Sans wants to avenge him. After a ridiculously hard fight with the most epic music ever, you eventually beat Sans only to hear his heartbreaking last line "Welp. I'm going to Grillby's. Papyrus, do you want anything?" Then, you go to the barrier and see Flowey, once known as the most ruthless killer of the Underground, literally begging you not to kill him. You kill him and see another human child who looks a lot like you.
This child is named Chara and was the first human to fall Underground. She hated humanity and wanted to kill everyone. Basically, she possesses Frisk's body and together they go to the surface. But then, she asks if you want to reset the universe. If you say yes, the universe is resetted. If you say no, the universe is resetted anyway.
The Neutral Route(s)
Okay. I'm going to be honest.
No one really cares about the Neutral endings.
Basically, it's just a bunch of different endings depending on who you kill. There are countless of these.
Media and Other Stuff
Undertale was and is super popular. What makes it unique is that, unlike most video games, it does not promote killing. Remember the LOVE and EXP? LOVE stands for Level Of Violence. EXP, which normally stands for Experience Points, actually stands for Execution Points.
The reason why Toby Fox is such a genius is because this game reflects real life. Everyone has a Chara inside of them.
Unfortunately, the Undertale Fandom and Community kind of...took a turn. The fanfictions became more inappropriate and poorly written, the fanart was inappropriate as well, and the abomination known as "If Undertale Was Realistic" was created. Now, there are still great fan pieces out there. Some of the best fanart I've ever seen is from Undertale. But it's the bad eggs that stick out and give the Undertale Fandom such a bad reputation. A lot of people I know describe it as "great game, cringe fanbase."
So I hope that summed things up for you!