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.