As Christmas approaches, an outbreak of measles hits Poplar. Meanwhile, Tom receives an intriguing offer from the BBC, a face from Patsy's past returns, and Sister Monica Joan goes missing.

As Easter approaches, the nuns prepare for celebrations, Trixie is taken up with a new exercise class, Patsy awaits Delia's decision, and the group are shaken by the birth of a severely deformed baby.

Barbara has difficulty in working out how best to help a young woman whose husband is out of work, Sister Evangelina's strong opinions lead to trouble, and Phyllis attends a Spanish class, with unexpected results.

One of Patsy's patients is diagnosed with typhoid, which reignites painful memories, Sister Winifred is deeply affected by the case of an unwed mother-to-be and Tom asks Barbara out to dinner.

Tom and Trixie work together to help a young couple, Barbara wrestles with her conscience, and Sister Julienne is seconded to a maternity hospital, where she discovers the emergence of a worrying pattern.

Phyllis becomes concerned about a new mother who is finding it difficult to cope, Fred must step in to help run the shop when Violet is injured, while Timothy makes Patrick have a realization.

As news of the forthcoming contraceptive pill reaches Poplar, it is met with different reactions by the residents of Nonnatus House. Meanwhile, Patsy tries to help a barge woman unused to life on land, and a storm hits Poplar.

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.