LaGuardia is the mayor of New York City, Paul Whiteman is the King of Jazz, and, up in his apartment on Riverside Drive, George Gershwin is asking his friends in to hear his newest composition, Summertime. On Fifty-second Street is the Onyx Club, a famous jazz club owned by ex-bootlegger Joe Helbock. Harry MacNeil, an ex-cop now working as a private eye, lives and works above the Onyx. But he hadn't been in the club on the New Year's Eve when, while Art Tatum was playing the piano, two of Owney Madden's boys came into the club and gunned down a two-bit mobster named Joey Seldes . . .