As Poplar prepares for Christmas, the nuns leave for the Mother House to vote in a new mother superior and to help care for a group of Chinese orphans. Meanwhile, Trixie returns to Poplar refreshed and is shocked to discover that many mothers no longer want to give birth at home.
A new midwife joins the team as the area is rocked by power cuts and blizzards. The new arrival is delayed by the weather and falls ill as soon as she arrives. However, she is quickly dragged from her sick bed to assist with a difficult birth. Nurse Crane and Dr Turner treat an elderly cancer patient who faces losing her house due to slum clearances, and try to convince Sergeant Woolf to allow her to die at home.
Valerie's friend Marjory goes into labour while Lucille is on duty, and the new recruit successfully delivers the baby. However, when Marjory's health takes a turn for the worst, her racist mother Mae accuses Lucille of incompetence. The nurses rally round to protect their new colleague, and convince her that she is welcome in the community. Sister Winifred comes up with a plan to get fathers more actively involved in their wives' pregnancies - but finds that very few of the local men are keen on the idea.
Nurse Crane and Trixie investigate when a mother abandons her children at the clinic, fearing they were being abused or neglected. Her husband admits that she has been behaving strangely lately, and when she is referred to a neurologist by Dr Turner, the truth proves devastating for the whole family. Christopher admits that Alexandra has been unhappy since his divorce, and Violet and Fred organise a beauty contest to raise money for charity. Valerie and Magda both sign up and quickly become close friends - leading to the sharing of a terrible secret.
Sister Julienne helps a woman from Pakistan who is unable to have children, but who has found to her shock that her husband is bringing a second wife into their home, who is already eight months pregnant. Trixie hits the bottle following her break-up with Christopher, Dr Turner tries to persuade Sister Monica Joan to admit that her eyesight is failing, and Lucille helps a woman with a compulsion to eat coal.
Rumours spread around Poplar that a sailor with smallpox is hiding somewhere in town. Nurse Crane and Dr Turner try to track him down before he causes an outbreak. Lucille treats an expectant mother on her own, but soon feels out of her depth when she displays signs of a serious medical condition. Trixie goes to stay with her godmother, Violet and Fred organise a picnic for the parish, and Tom and Barbara return home.
Barbara helps a pregnant mother-of-two whose husband has recently died, and is on hand to give her support when a further tragedy drives her and her children from their home. Lucille teaches a health and relationships class at the youth club, only to encounter a furious mother who objects to her daughter attending. Sister Monica Joan goes into hospital for cataract surgery.
Barbara's condition is not improving and she is kept in isolation at the hospital, leaving her friends able to do nothing but wait for more news. Dr Turner and Shelagh assist a man being held in a remand home whose wife is pregnant, while Lucille makes some new friends.
As the nurses and midwives of Nonnatus House prepare to celebrate Sister Monica Joan's birthday, they face an influx of new patients when a nearby maternity home suddenly closes, leaving many expectant mothers in urgent need of support. Lucille treats a woman whose father has a long-hidden secret he feels he cannot hide any longer, while Tom and Nurse Crane struggle to move forward with their lives.