“The Great Society rests on abundance and liberty for all,” said President Lyndon Baines Johnson in a speech of 1964. “It demands an end to poverty and racial injustice.” Robert Schenkann’s latest play, named for this landmark speech, is a sequel to his Tony-winning All the Way, which depicted President Johnson, played by Brian Cranston, strategically working to pass civil rights legislation. Brian Cox takes on the part of an older L. B. J., and the story picks up after Johnson’s reelection by a landslide in 1964. —C.J.F.