As the nuns and nurses go about their duties, Nonnatus House gains a new resident, Chummy, a welcome pair of hands to help with the busy caseload. Sister Evangelina, in a case of inverted snobbery, has little patience with her or with her passionate and somewhat lofty desire to become a missionary.
Meanwhile, Jenny has a chance late-night encounter with Mary, a young Irish girl aged 15. She is heavily pregnant and reveals that she ran away from an abusive home life in rural Ireland and made her way to London. She is unwilling to give away many details of her current situation. Mary is desperate and terrified, and eventually Jenny discovers the full, horrific details of the young girl's plight.
Destitute upon arrival in London, Mary was "taken in" by Zakir, the owner of an all-night cafe in the notorious Cable Street area. It transpired that the cafe was in fact a brothel, and Mary was forced into prostitution almost immediately. Her suffering has been extreme, but in the tough world of the criminal East End, this is far from unusual.
Sister Julienne arranges for Mary to be helped by a religious charity, but sadly it is not to be the end of her tragic story.
Chummy makes great efforts to settle in, but nerves and a keenness to please almost overwhelm her, and she makes slow progress. A particular obstacle is her inability to ride a bike, and although Jenny, Trixie and Cynthia struggle to teach her, their efforts are hampered by catcalling and jeering from local children. Her initial attempts to learn end in a spectacular collision between the three midwives and a policeman, PC Noakes.
Chummy's future at Nonnatus House hangs in the balance, and she soon finds herself out of her depth.