The unwitting recipient of a destructive prosthesis -- a powerful blaster grafted onto his left arm -- John Cain awakes to find himself being hunted down by a military organization, with no recollection of the events leading to this predicament. Inspired by Contra and other such titles, One is a pseudo-3D scrolling shooter in which players find themselves traversing a variety of locales, from futuristic Blade Runner-esque cities to remote mountainous regions, all in search of answers. The central gameplay mechanic is the player's Rage meter. Simply put, John Cain is fueled by rage. By destroying objects, killing people, and generally causing all kinds of wanton mayhem, he will increase in power. Cain's rage is represented by an orb-like indicator that undergoes a number of color changes, from the relative calm of green to an infuriated red, and beyond.