When a Brooklyn high school student writes the police code for homicide, 187, inside a textbook owned by teacher Trevor Garfield, Garfield feels threatened. The principal dismisses the incident, but the same student stabs Trevor soon after. Fifteen months later, a physically and emotionally scarred Trevor relocates to California and takes up substitute teaching. To his dismay, his new school is as full of dangerously undisciplined students as his old one—including four Latin Americans and a European American claiming membership in tagging gang K.O.S. (Kappin' Off Suckers)—driving him over the edge.