At its heart, this tiny game tells a straightforward story, but it's the context of its video game setting that makes it so special.
Undertale (in case you haven't been on the internet in the last five years) follows a young boy called Frisk who falls into The Underground, a realm full of monsters, and from there, he ventures to find his way out.
But that isn't the real story here. No, while that is the core experience, the beauty of Undertale comes from the changes that occur based on your actions. The game's notorious branching paths - neutral, pacifist and genocide - create entirely different sides to the story. Your decision to spare or kill monsters leads to massive ramifications, but where that becomes truly special is in the game's meta-narrative.
In Undertale, each of your saves and resets are canonical. If you decide to experience a pacifist run and receive a happy ending, your choice to start the game again is actively still a part of that same world. By resetting, you rip that happiness away from the characters you've grown to love, and this works in reverse too.
There's no way a concept as inherently tied to your decisions like that could work anywhere but in a video game.