Patrick is horrified to learn that a medication prescribed to expectant mothers may be the cause of the recent birth defects in Poplar, while the Nonnatus House residents and the community at large are devastated when tragedy strikes.
The Turners attempt to improve the life of a young man from a remand home, Lucille finds unexpected friendship, while Nonnatus House is filled with worry for Barbara.
Chummy struggles to cope with her mother's terminal illness, with her experience affecting Sister Monica Joan greatly. Meanwhile, Shelagh and Patrick receive encouraging news, and Jenny makes an important decision.
Nonnatus House faces its darkest day after a tragic incident in the heart of Poplar. As the community rallies together, the team must overcome personal anguish to help the injured.
Dr Turner experiences the biggest test of his career, while Trixie's approaching nuptials cause stress levels to escalate. Sister Julienne hatches a plan to save Nonnatus House.
Sister Bernadette is discharged from hospital, while a heavily pregnant Chummy resumes her midwifery duties. Jenny meets a new love interest, Fred's pregnant daughter pays him a visit, and Nonnatus House itself is under threat.
Sister Winifred treats a young Jewish woman whose mother is plagued by remembrances of the Holocaust. Meanwhile, Shelagh tries to find a new way to channel her energy, and Alec suffers a terrible accident.
A series of attacks on young women in Poplar has a direct impact on the residents of Nonnatus House, Trixie is suspicious about a patient whose pregnancy has not been reported, while Patrick and Shelagh take a trip.
Nonnatus House and the surrounding area is evacuated when an unexploded bomb is found nearby. Meanwhile, Shelagh has doubts about her wedding to Patrick, and Jenny and Trixie help a man who is suffering from shell shock.