A cop is killed and a mob boss is wounded in a shoot-out at a nightclub where young Dr. Tom is dining with his bride-to-be, Mary. After being asked to treat Boss Grusso's injury, Tom is approached by police to go undercover as the gang's "in-house" physician in an effort to bring them to justice. Tom's new secret life puts a strain on his relationship with Mary, and he's determined to get out of the dangerous assignment. The police chief, sensing a break in the case, convinces Tom to stay on. Neither of them is aware that Tom's cover has already been blown, and that Grusso and his thugs have laid a deadly trap.<P>Richard Cromwell followed up his <i>Riot Squad</i> performance as Dr. Tom with the title role in the 1942 gangster film, <i>Baby Face Morgan</i>. Screen veteran John Miljan, <i>Riot Squad</i>'s oily mob boss, appeared in 200 films in his three decade career, most memorably as General Custer in Cecil B. DeMille's 1936 Western classic, <i>The Plainsman</i>. <i>Riot Squad</i> also marks the last screen appearance of eight year old Mary Ruth, once tagged as "the next Shirley Temple," seen here incongruously interrupting the crime drama with a song.