After his schoolteacher girlfriend is assaulted by a student gangbanger, mercenary Shale goes undercover, entering her classroom as "The Substitute" with the intention to weed out and teach a lesson to her attacker. Thrust into Miami's drug-infested and gang-ruled urban school system, Shale gets in deeper than he planned, uncovering a city-wide web of corruption, armed thugs and professional drug trafficking. Along with his mercenary team, he is pitted against the powers poisoning the school, and an entire high school full of kids is caught in the middle. The stakes for Shale have never been higher.