WBLA is on the air, presenting the live music, the sudsy dramas and the sell-sell-sell of commercial interludes that keep consumers buying and sponsors smiling. But one sponsor, a producer of plumbing supplies, isn’t happy. So WBLA scriptwriter Bill Grimes is bounced from his job, setting in motion this movie’s turn from comedic to darkly tragic. William Haines, two years removed from being Tinseltown’s top male star, plays Grimes in a melodrama noted for its glimpses of live radio production and for a Depression-era ethos that includes peroxide cuties eager to land a job, a sugar daddy or both.