Undertale has one of the most gripping game plots I've ever played in a game. It's a unique take on the hidden object genre and has solid mechanics that are incredibly tight, but also have enough charm to make you play it repeatedly. The premise is simple: you're playing as Frisk, an orphan whose goal is to find his long lost brother. To do so, he must travel through many different environments and rooms in order to find his brother, as well as an endless amount of obstacles that will stand in his way. There are a few different paths to follow throughout the game, and each has its own unique ending.
Unfortunately, some people really hate the twist in Undertale sans Mark. In this version of the game you basically get one chance to get into the final fight with your brother before he vanishes, so you basically just end up making the same mistakes over again. It's unfortunate that this one part of the game is left out, as it's a very good game overall, but it's just a minor nitpick.
Despite this minor glitch, Undertale is still an outstanding game. It has excellent graphics and music, as well as some of the best puzzles and adventure gameplay I've seen in a video game. If you like hidden object games with a strong mystery element, then you'll love Undertale. If you're a fan of horror game plots, though, you might not care too much about this one bit. Regardless, it's a great game to play and you should definitely buy Undertale.