Going under cover, P.C. Mahoney passes for a gentleman to get into the notorious Moonstone Club. There he meets Clifford Tope, a ne'er do well who is love with cabaret star Cora Mellish. She in turn has run up steep gambling debts and has paid off the Club's blackmailing owner with a stolen necklace. As things heat up Cora seeks help from the easy-going Tope.