A cast of 20. An original story, not based on a book or movie. Plenty of dancing. Few modern musicals have all these things, and that Only Gold does indicates the breadth of its ambition. Set in Paris in 1928, the show features an ensemble in near-constant motion on an art deco–styled stage with a long, winding staircase whose banister extends into a circular fixture suspended above the stage amid a sky of globular lights. In design and concept it’s a very ambitious project indeed.