In the paradoxically plaintive and joyous sound of a castrato's voice channeling Handel's music, the King has glimpsed a paradise beyond his fractious court and his burdened royal self. Trying to create that idyllic vision in the real world, in a rustic outpost in the forest in the second act, is an experiment doomed to failure. But watching Mr. Rylance's Philippe experience Farinelli's voice, we hear what we hears. And an actor and a singer temporarily turn a night at the theater in an anxious city into an Eden beyond worldly care, all the more precious for its evanescence.