The Crown’s episode seven of season four, “The Heredity Principle,” begins with what at first seems like a non-sequitur: two women in a mental health hospital, watching Queen Elizabeth wave to crowds on television. When the broadcast begins, they stand up, with one of them even saluting the screen. Throughout the first 25 minutes of the episode, scenes of these aging, disabled women are interwoven with those of Helena Bonham Carter’s Princess Margaret, who begins to see a therapist for the first time. Then, it’s all connected. “Are you aware of anyone else in your immediate family struggling with mental health issues?” the therapist asks Margaret. “I only ask because I am aware, through professional colleagues, of the sisters.”

The sisters, as it turns out, are the two frail mystery women we’ve seen all along: Nerissa and Katherine Bowes-Lyon. First cousins of Margaret and Queen Elizabeth, they’ve been forgotten by even their own family: “But they’re long dead!” the queen exclaims to Margaret over lunch after she mentions their names. Together, they search through a copy of Burke’s Peerage, the who’s who of British aristocracy. Indeed, it lists both as deceased. An incognito Margaret drives up to Royal Earlswood Hospital in Surrey and sends her friend in on a reconnaissance mission. When he comes back, he confirms what Margaret suspects: Nerissa and Katherine are there, along with several other relatives.

Later, Margaret confronts her mother—and Nerissa and Katherine’s aunt—about their institutionalization. “My family, the Bowes-Lyons, went from being minor Scottish aristocrats to having a direct bloodline to the crown, resulting in the children of my brother paying a terrible price,” the queen mother says. “Their illness, their idiocy and imbecility, would make people question the integrity of the bloodline. Can you imagine the headlines if it were to get out?”

Like most of The Crown’s plotlines, the episode is based in a sad reality. Nerissa and Katherine Bowes-Lyon were indeed sent away to Royal Earlswood Hospital in 1941. At the time, Nerissa was 22 years old and Katherine just 15. Although their exact diagnosis was unknown—at the time, they were just called imbecile—the sisters were said to be severely handicapped and nonverbal and have a mental age of six. They remained in hospital care until both of their deaths. (Nerissa in 1986 and Katherine in 2014.)