From the introduction:
Role-playing games are a type of cooperative game where players each control an individual character and another player, the game master, controls the rest of the world. Unlike traditional games there is no board, and no pieces. Instead, players describe the actions of their characters and then the game master tells them the results. Often times there is a random element, be it dice or cards, that allow for exciting outcomes that are often unpredictable.
Overkill is like this, with the central conceit being that players are controlling characters in a world that resembles an action movie, and the game master is the director, creating enemies and scenarios that allow for the realisation of every child's fantasy, to be the good guys and save the day. Of course it doesn't have to be that way, and there are a variety of themes and scenarios available, though most of the paths, feats, and items expect this sort of game. Feel free to adjust any numbers or themes, to create a more gritty or bombastic experience.