Cynthia (later Sister Mary Cynthia) goes through one of the darkest character arcs in Call the Midwife , and then is quietly written out rather than given an on‑screen farewell.

On‑screen story: what happens to Cynthia

  • She starts as Nurse Cynthia Miller, a gentle, anxious but very capable midwife at Nonnatus.
  • Feeling a religious calling, she takes vows as a nun and becomes Sister Mary Cynthia, remaining at Nonnatus while still midwifing in Poplar.
  • She faces early trauma when a baby she delivers dies and she is investigated by the police, leaving her full of self‑doubt until she’s cleared of blame.

The attack and PTSD

  • While cycling home after a night call, she stops to pray on the pier and is violently attacked and knocked unconscious by a man who has been targeting women.
  • She wakes battered, with bite marks on her neck; when the others fear she was raped, she angrily insists she was not, then retreats into herself and becomes jumpy and hypersensitive to touch.
  • Although she eventually reports the attacker and helps the police, she later has a severe PTSD flashback while protecting a mother and children from a violent husband, which triggers a deeper mental‑health crisis.

Sent away and institutionalised

  • Officially she is “sent to the Mother House to convalesce,” but it emerges that she has instead been placed in Linchmere Hospital, a cold and harsh mental institution.
  • There she is subjected to electroconvulsive therapy (ECT), against her will in some accounts, which leaves her confused, depressed, and unable to “hear God,” with staff interpreting her faith as delusion.
  • Sister Monica Joan and Fred find her by chance when they visit Linchmere for Reggie; Sister Julienne and Dr Turner then fight to have her released to a more humane place of care.

After Linchmere

  • She is eventually moved to a different, more therapeutic mental hospital, the same kind of environment that helped Dr Turner after the war.
  • Even after release from Linchmere, she continues to struggle with severe depression and guilt about not being “a good nun,” and it’s implied she needs long‑term rest and treatment away from Nonnatus.

Does Cynthia die? Is her story finished?

  • Cynthia does not die on screen; fans sometimes assume a “demise,” but viewers and forum posters repeatedly clarify that she’s alive, just off‑screen in treatment or recovery.
  • The series never gives a neat, narrated wrap‑up for her: no postcard, no explicit “she recovered and now teaches midwives” scene, which many fans feel was a missed opportunity.
  • Because of that ambiguity, some viewers read her arc as an open‑ended struggle with trauma and faith, and others like to imagine she eventually heals and returns to midwifery or religious work in a healthier setting.

Real‑world reason she disappeared

  • Actor Bryony Hannah chose to leave Call the Midwife around 2017 after about five years on the show, to focus on other work, including theatre and later appearances like Death in Paradise.
  • Her exit meant the writers phased Sister Mary Cynthia out after series six, without bringing her back in later seasons, and her fate is left largely to implication rather than explicit closure.

Information gathered from public forums or data available on the internet and portrayed here.