In his first starring role (after being second-billed to Claudette Colbert in The Gilded Lily), Fred MacMurray plays officer Ross Martin of the Michigan State Police. After completing his training, Martin is pitted against dignified Professor Anthony (Sir Guy Standing), who uses his academic status as a cover for his bank-robbery activities. Keeping himself abreast of police maneuvers by listening to car radios and unobtrusively hanging around headquarters, Anthony ultimately uses his technological know-how to paralyze the police communications systems. But with the cooperation of the Massachusetts police department, whose radios are in full working order, rookie Martin and rustic sheriff Pete Arnot (Frank Craven) combine forces for a final assault upon Anthony's hideout. Its sometimes illogical plot twists notwithstanding, the screenplay is based on a series of factual articles, first published in Saturday Evening Post. Also given a career boost in Car 99 is another new Paramount contractee, Ann Sheridan.