Baptiste and Genevieve search out the Romanian gang's inside source in the Amsterdam police while Edward continues to try to help Natalie's kidnapped sister, Cristina, and the investigation winds to a close.
In the season finale of World on Fire, Robina tries to come to an understanding with Harry and Lois, Lois considers an offer from Vernon, Harry parachutes into danger, and Nancy tries to help Webster in Paris.
McNulty must help a woman whose husband is a drunk, but the young doctor has his own issues, which worry Sister Frances. Trixie helps a blind mother who rejects any attempt at help and is in a spat with her sister.
Martha's life begins to crumble as Genevieve steps into the investigation into Constantin, while Baptiste continues to search for the money and Edward is pressured from all sides, leading him to make a drastic decision.
The Turners' adoption of Mae hits a difficult obstacle, while a relative of Valerie's faces a sorrowful tragedy after the birth of her third child. Fred works to put on Poplar's annual horticultural show.
As Baptiste learns the truth about Edward, his family is put in danger. Europol becomes involved as Baptiste works more closely with the Amsterdam police, including Martha and Niels. And more is revealed about Natalie.
Kasia is driven to ever more drastic actions by her experience of cruelty in occupied Warsaw, while Harry's leadership is tested as his unit engages in combat in Belgium and Douglas struggles to help his children back in England.
Sister Hilda and Nurse Crane have differing opinions about the involvement of a husband in his wife's pregnancy, while Sisters Julienne and Frances become involved in a middle-aged woman's life and end up re-examining their views of themselves.
Nancy begins digging into nefarious Nazi programs while Tom engages with a German warship in the south Atlantic. Harry and Lois meet again at Harry's infantry base. Robina tries to make Jan happy in a foreign country.
Fred and Reggie meet an ailing man who keeps pigeons, and try to convince him to move out of his condemned building while helping with his health. Four young doctors move into Nonnatus House to learn from the midwives.