S4E7 · The Hereditary Principle

With Edward coming of age, Margaret finds her public role reduced and she falls into depression. She travels abroad to recuperate from having part of her left lung removed, and starts seeing a therapist on Charles's advice. After the therapist inadvertently mentions her deceased maternal first cousins Nerissa and Katherine Bowes-Lyon, Margaret learns that they are still alive and, along with three other cousins, in a mental institution. She confronts the Queen Mother, who claims the family had no choice as knowledge of the cousins' existence would have called the purity of the bloodline into question. Margaret confides her insecurities about becoming mad to her therapist, who reassures her she will not. Margaret reassumes her role yet broods in private.