Buy it. Fucking buy it. You will hate yourself if you don't. It's only ten bucks on Steam. It's worth every penny. Don't believe me? Then explain to me why it's got a 97% rating on metacritic? Go get it. Go play it. Don't look at spoilers. In fact, avoid Youtube and Google entirely. Just go buy it, beat it, and talk to me when you're done.

TL;DR Buy Undertale. You won't regret it.