Screenshot taken from the game
The story starts out with a child of ambiguous gender climbing Mt. Ebott and falling through a hole. Said hole leads into some underground ruins part of the world of monsters, a civilization of people banished below the earth after losing a war against the humans long ago. While there you meet a woman named Toriel who adopts a motherly attitude towards our hero and explains the many dangers of this new world. At first it seems like a typical "explore exotic locales, fight bad guys and return home" type of plot, but early on in the game you encounter some monsters in battle who seem...different. Ones who seem afraid of you, or apprehensive of your presence. Beyond the commands for 'Fighting' in battle, you have the option of 'Sparing' weakened monsters or interacting with them via the versatile 'Act' command to come to a more peaceful solution. But not all monsters you meet are so reluctant to leave a human unharmed? It's one thing to say you won't fight and will show mercy: can you still do as much when in the face of certain death?
Attempts at subversion of common tropes don't always end well, but the way Undertale pulls it off makes it much more interesting than just a postmodern commentary on the violence-centered aspect of RPGs. You can definitely play it hack and slash style, or try your best to be a pacifist in spite of the many dangers laid in front of you. The best thing is that Undertale has great replay value and lots of hidden secrets, with even minor choices having resonating effects throughout the plot. The characters are all well-detailed, and it's one of the few games which had moments where I burst out laughing or a sense of remorse at seeing a character I grew to like over time come to a bad end. When I first bought it I found myself playing it for 6 hours in a single day, so engaged was I with everything about it that I couldn't wait until tomorrow to see what happens next.
The game has good replay value; as of this post I'm on my second run, and the story isn't linear even if a lot of the locations you visit are the same. The events and characters change in major ways depending on your actions. A monster who you spare in a random encounter might show up later as a person to talk to, whereas ones you kill might come back to haunt you as their friends show remorse on losing a loved one. But it's not quite a simple "black and white" moral choice like in most mainstream games such as Dragon Age. It takes major effort to go the route of either a pacifist or remorseless killer, and Undertale's atmosphere and plot changes where it often feels like you're playing a different game entirely: there's a rather significant number of players who simply can't find themselves the willpower to go through the 'evil path,' or "No Mercy" run. I'm starting to find myself in their camp as well as time goes on.
In conclusion, Undertale is a great game which made me care for the people of its world far more than many other RPGs on the market. It has a free demo, and can be purchased for ten dollars on Steam's online store. It has the potential for being one of 2015's top games in spite of being an indie-published. I highly recommend you at least give it a shot.