Susan Traherne, the heroine of David Hare’s 1978 drama Plenty, is hard to love. She has a tendency to pop off on matters of social and political delicacy, an inconvenient trait for the wife of a British diplomat in the years following World War II. She exudes disappointment in a proscribed life with a weak man, neither of which can ever live up to the excitement and promise she felt as a courier in the Resistance during the war. That disappointment has driven her to…