This is the first time that the character of Cathy does not appear in an episode. She is not mentioned and no explanation is given for her absence.
The play that is being discussed by Patty and Bob, played by Steve Franken, is George Bernard Shaw's "St. Joan," which premiered in 1923 on Broadway. Since then, it has been produced several times as a film, either as a TV broadcast or as a release for movie theaters. Jean Seberg starred in the titular role eight years prior to the original broadcast of this episode of The Patty Duke Show (1963). Two years later, Geneviève Bujold would star in the titular role for an NBC-TV production.