You are a group of monster hunters, either teenagers in a small town or a group of paramilitary/police recruits, who must track down and kill/capture these creatures. This process includes hunting down clues about the monster, finding its weaknesses, tracking it down, and capturing or killing it.
This would be more episodic, with each monster hunt a self contained experience, but part of a larger plot as a whole. Think shows like Buffy the Vampire Slayer, The X-Files, and Supernatural (or other CW trash TV), or even police procedural shows where they need to hunt a new killer every week (a la Bones, Law & Order, etc.).
The system would be run using Monster of the Week, a Powered by the Apocalypse system which is much more free form than D&D.
Monster of the Week (MotW) is more free-form and player driven than D&D. In D&D the Game Master sets up a scenario, puts you in it, and then does things to you in order to move the story or scene along. In MotW the Game Master sets up a scenario, puts you in it, and then reacts to things based of your rolls or decisions. They very rarly (if ever) directly intervene when not in response to your rolls. This can be explained in further detail later, if this game is ever run. The system itself uses 2d6 for all roles, and your success on an action is based off the result of a die roll (modified by one of your stats: Cool, Tough, Charm, Sharp, & Weird, each rated from -1 to +3)
If it seems like you're set up to fail, you are. Fighting monsters is hard, and you will likely fail your rolls, but that is also how you gain XP to level up.
The base rules have 12 classes. A brief overview below. I would only allow one of any class.