Policeman James Aloysius O'Malley knows the law better than any other policeman. He is constantly issuing citations for obscure laws that are generally no longer enforced. For this reason, the people on his beat dislike him and his fellow police officers make fun of him, but O'Malley is convinced that the law is the law and must be enforced exactly as written, even if it causes trouble for otherwise law-abiding citizens. His mother encourages him to follow in his father's strict footsteps until he stops her from throwing breadcrumbs to the birds because that would be littering. O'Malley stops John Phillips for a noisy muffler, and even though Phillips agrees to fix it, begging O'Malley not to make him late for his job, O'Malley insists on writing him a ticket. Phillips, whose daughter Barbara is crippled, loses his job and is forced to pawn his army gun and war medals. When he cannot get the money he needs, he robs a store, but is quickly arrested by O'Malley, who stops him again for his noisy muffler. After Phillips is sent to prison, despite his lawyer's pleas for mercy, O'Malley is demoted to school crossing duty in hopes that he will learn to temper the law with human feeling. While working, he becomes friends with Barbara, not realizing that she is Phillip's daughter. When he learns her identity, he is guilt-stricken and secretly arranges for Barbara's leg to be fixed, secures Phillips' release from prison and convinces his former employer to rehire him. Not knowing of O'Malley's efforts, Phillips expects to be hounded by the policeman. When O'Malley appears at the Phillips' door, Phillips shoots him in a panic. Then when he learns the truth, he offers his own blood for an emergency transfusion to O'Malley. O'Malley recovers and returns to his old beat, but this time, he is more reasonable. He becomes popular with the neighbors, and Judy Nolan, Barbara's teacher, falls in love with him.