Proof is the story of a young woman named Catherine. She is the daughter of Robert, a brilliant mathematician, who misplaces both his brilliance and his sanity in his later years. Catherine, a budding mathematician herself, must give up her schooling and her most creatively productive years in order to take care of her father, who has become convinced that alien civilizations are communicating with him directly through the local library's Dewey decimal system.

As the play opens, Robert has just died and Catherine, conversing with his ghost, wonders if she may have inherited his tendency toward madness. Robert had first shown signs of madness in his mid-twenties. Catherine has just turned twenty-five. Her suspicions seem confirmed when her sister Claire and Robert's protégé, Hal, begin to treat her as if she is mentally unstable. Claire, returning for her father's funeral, wants to...