Kirsten was a young woman, daughter of a cargo ship captain, who was brought along on the trip solely for the purpose of sexually servicing the crew. Her father told her that there is "no fighting on a ship when the men have a woman they can go to for relief". Because her father never let her run out of condoms, and because she was heavy-set, Kirsten did not know she was pregnant until the nurses came and confirmed it.
After Constable Noakes gets a call that a woman is in labour aboard a Swedish Cargo ship, he calls Nonnatus House and informs Chummy of what was reported. Knowing that women are not permitted on cargo ships, and wary that it may be a trap to lure a defenseless woman aboard a ship full of men, Peter informs Chummy not to send Trixie on her own to the ship.
Jenny leaves to tend to another one of her patients, so Trixie goes to the docks with Sister Evangelina. After a harrowing experience rowing out to the ship and then climbing the rope latter onto the deck (wherein Sister Evangelina dislocates her shoulder) the two are relieved to see there is indeed a woman in need of assistance.
Kirsten is then examined and her pregnancy is confirmed, much to her surprise, though she says her father expected it. Somewhat clueless (as though she sees nothing amiss with the idea), Kirsten reveals that she is there because her father prostitutes her to the shipmates.
With limited room in her quarters, Kirsten is moved to the captains cabin. After another examination, Trixie discovered that the baby's cord has prolapsed, meaning the umbilical cord has been pushed into the birth canal, and if it goes untreated, the baby's oxygen will cut off and it may die. Though a c-section is preferred in these circumstances, it is impossible on a ship, so Trixie improvises. Using a chair to elevate Kirsten's pelvis, she uses her hands to push back the baby just enough for the cord to slip back.
Kirsten gives birth to a little girl, though it takes the baby a while to cry. Hearing the baby's cries outside the door, the ship's crew erupt into cheers and begin to sing.
Kirsten falls in love with her child the moment she sees her, and realizes she wants a better life for herself and her daughter. She says "I have lived without friendship for such a long time. But today, you [meaning Evangelina and Trixie] came. And she [baby] came, too. So now I have a friend forever."
When Trixie and Evangelina leave in the morning, Trixie tells off the captain, and says he'd better have a boat ready for her everyday for the next two weeks so she can examine Kirsten and make sure she and the baby are in good health. She shames him for the way he's exploited his daughter.
On the last day of Trixie's two week post-natal check ups, Trixie gives Kirsten a gift of fine quality condoms. Kirsten reveals she is done with ship life, and is returning home to Stockholm, so she and her daughter will have a better life. Kirsten says she told her father this, and he agreed, to which Kirsten says was because he was ashamed of himself. She says that for the first time, the Captain and his daughter saw themselves through Trixie's eyes. Kirsten says becoming a mother has made her stronger.
The last we see of Kirsten, she is sailing away, looking back at the UK as it fades away.