Play it. (Warning: the game is not as silly and cutesy as it looks. Also, to exit it, you have to press and
hold Escape. Just a heads-up. And, gameplay tip, in the very first room where you have control of the player character, the exit is not obvious. Go to the
right.)
http://undertale.com/
Listen to the soundtrack, too. (IMO, track 12 is the best; your mileage may vary.)
http://tobyfox.bandcamp.com/