On the Twentieth Century follows a down-and-out Broadway producer named Oscar Jaffe, who struggles to convince his former muse and lover, Lily Garland, now a successful film actress, to return to Broadway in a (non-existent) epic drama about Mary Magdalene. While dealing with Lily Garland's jealous new lover and a religious fanatic aboard a luxury train, Oscar hopes he can lure her back to the stage and salvage his sinking career.