There are no scene or costume changes, or props. “It moves through time,” says the playwright Duncan MacMillan, “in a way that breaks the conventions of what people expect from a play.” Yet Lungs asks that most eternal of questions: Why would you bring a child into a world of such political and environmental trouble? This production travels from London’s Old Vic Theatre and stars Claire Foy and Matt Smith, who were partnered as Queen Elizabeth and Prince Philip in the first two seasons of The Crown. —L.J.