Brock attempt to meet the traditional social responsibilities of men in that Mick tries to father a child with Susan and become involved with her while Brock later cares for Susan financially and emotionally as her husband. Interestingly, many of the characters in this play can be described as failures in these respects.

Although Susan feels that she is helping France curtail Germany's encroachment during the war, the Frenchman that she and Lazar encounter in the first scene tells them that the English are not welcome in France. From the Frenchman's perception, Susan and Lazar are fulfilling duties that are not necessarily required of them. After the war Susan fails to adhere to the unspoken civil responsibility of following social protocol. Instead of being quiet and acquiescent (as women were expected to do), Susan is often verbose and brash. Susan and Alice (who adheres to social mores even less than Susan) seem almost unpatriotic in their dislike for their nation, its policies and conventions. Susan also refuses to let Mick assume the responsibilities that being a lover, father, or even a friend would entail—despite Ms apparent willingness to be all of the these things to her.

Darwin relinquishes his duties in the foreign service in order to be honest with himself and others about what he perceives really happened over the Suez Canal. By abandoning the socially acceptable diplomatic role, Darwin refuses to take responsibility for the lies of his country. Ironically his honesty codes him as unpatriotic in that he does not unconditionally support England. For different reasons Brock fails to meet his professional responsibilities in the eyes of the foreign service. He does not advance through the ranks because he fails to excel in his duties. At the play's conclusion, he is working in the insurance industry. Ironically insurance protects people when and if they are unable to care for themselves. Thus it seems that while on the surface this play focuses on the ways in which the characters meet their duties and responsibilities, it also turns this notion upside down by exposing the ways in which they fail themselves and others as well.

Sanity vs. Insanity
Another central theme of Plenty revolves around the question of Susan's state of mind. Towards the end of the play Susan admits to Lazar that she has "not always been well"; however, just after this admission she also tells him that her clarity of mind is something that she controls. This scene suggests that, as Susan says, she simply "likes to lose control" at times. While "losing control" may have been an asset during her daring wartime work, it is a less desirable trait in England's staid postwar society.

In an earlier scene Brock denies that Susan is mad by suggesting that she simply "feels strongly." After Darwin's pressing however, Brock admits that Susan does have a history of mental illness. Brock actually admitted Susan to a mental institution after the shooting incident with Mick. Despite the apparent proof that Susan is mentally disturbed, the scenes which betray this instability are also marked by Susan's willingness to speak what she believes to be true. Some critics have suggested that, with the character of Susan, Hare intertwines truth and madness. As far as his protagonist is concerned, the truth and insanity are mutually dependent traits; her mental instability fuels her need to expose the truth.

Truth and Lies
The question of honesty permeates this play and most of its characters. Susan exemplifies someone who is willing to speak her mind despite the fact that what she says may be inappropriate and offensive. Despite her devotion to the truth, Susan is not above deception. In fact, her work hi France was based on duplicity. Likewise, she does not intercede with the truth regarding the money she loans to Dorcas Frey. Alice tells Brock that the funds are for a hand operation while Susan knows—but does not betray—the truth: that Dorcas needs the money for an abortion.

Brock is another character who lies throughout the play. He lies to Tony Radley's widow about Tony traveling alone at the time of his death— although this he does serve to protect both Radley's reputation and his widow's feelings. Later, Brock seems to not understand Darwin's consternation regarding the lies that surround England's role in the Suez Canal incident. Although the play does not explicitly incriminate M. Aung as a liar, his exaggerated deference seems insincere. His wife, though perhaps not a liar, does convey false information when she tells the dinner guests that Ingmar Bergman is Norwegian when in fact he is Swedish. Darwin, who is associated with truth throughout the play, corrects this false information. Later Darwin resigns because he is unable to live with the he that continued diplomatic service might require. He feels betrayed and takes the higher moral ground. Darwin's morality is apparently respected by few of his peers, as evidenced by the small turnout at his funeral.

Hare seems to be criticizing the social practice of rewarding those who advance by any means necessary. While the characters who manipulate the truth are often successful, they are hardly respectable people. Although Susan is, for the most part, devoted to truth, her motivation is in part driven by bitterness over the lost hopes and dreams of her adventurous youth. Her obsession with exposing the truth also reveals her mental problems and may eventually lead her to imprisonment in an institution. Darwin, perhaps the most honest character in the play, is rewarded for his integrity with vilification. It's interesting to note that he shares the surname of Charles Darwin (author of the ground breaking Origin of the Species), the British naturalist known for advancing the theory of evolution. Darwin the scientist was frequently attacked for challenging preconceived notions regarding man's origins.