It has you playing as a kid who falls into an underground world of monsters. You have to find your way out or remained trapped there forever. And while you can fight all the monsters you find you don’t have to. In fact, it’s very possible to do a pacifist run of the entire game.
If you do choose to enter fights, you’ll find that the battle system is unique, as instead of being turn-based combat or hack ‘n’ slash it plays more like a shmup. The idea is to dodge projectiles fired at your heart (which is trapped in a box) and then time your attacks to dish out damage.
Alternatives to fighting include making friends with all the monsters by hugging them, including the biggest and scariest bosses, and even taking it further and going on dates.
If you want to give Undertale a try before it comes out then you can download the demo on its website.