The novel is divided into three parts and an epilogue. In Part I (four chapters), the two sisters have been living in an orphanage for almost a year, because their parents, who are fighting on the Republican side, do not want to leave them with relatives who have more Francoist leanings. Their parents decide to send them to Russia together with other children for their safekeeping. The children travel to Gijón harbour, where they board a French cargo ship. Their father is there to see them off, but their mother arrives late and is unable to see them. The children are taken to a children’s home in Leningrad, where they are looked after by a Spanish woman, María do Mar, and receive toys, sweets and colouring books. In her mind, Harmony composes a letter to her mother, explaining that they weren’t allowed to leave the boat to see her and it wasn’t their fault. She notices a boy called Leo, who seems to be without any relatives and who doesn’t cry on the journey to Leningrad, even though he’s seasick like everybody else. On arrival in Leningrad, he receives a carousel with horses and a cockerel, but it falls on the ground and breaks. This makes him very sad and he is taken off to see the doctor.

In class, the teacher invites the children to write letters to their relatives. Harmony is flustered when she learns she is actually going to have to write the letter she has been composing in her mind for days on end and doesn’t know where to start. In the end, she writes a quick letter, saying it wasn’t their fault they couldn’t come out to see their mother and they understand their parents are fighting for a just cause. Rose includes a drawing of a red hen surrounded by chicks to go with the letter. Harmony is told Leo will be returning from the hospital soon. She tells the teacher that they are friends because she feels a close connection with him, but then she is afraid that Leo will give her away when he returns and say he doesn’t know her at all, and the children will tease her and say she did this because she likes Leo. She also has a nightmare about the letter they sent their parents like a pigeon, being attacked by soldiers and dogs, and barely managing to stay airborne. When Leo returns to the school, however, he greets Harmony by name, and Harmony realizes he’s remembered her name from the ship, when the people from the Red Cross called out to her to say her mother was on the quay.

In Part II (seven chapters), the Republicans take control of a city, where among the prisoners Miguel, the girls’ father, comes across a friend of his, Henrique, the son of a Nationalist general, with whom he did teacher training. He can’t bear to let him be shot, so he frees him from the town hall, where the prisoners are being held, and helps him leave the city with his daughter. Henrique gives him a letter addressed to his father, the Francoist general, should the tables be turned and Miguel find himself in need. Miguel, however, is killed during the Battle of the Ebro. The children’s mother, Carmiña, receives his belongings, including the letter addressed to Henrique’s father. She is due to be shot, but instead is taken to the general’s office. The general, realizing what Miguel did to save his son (even though Henrique and his daughter were later killed by a grenade that was thrown at their car), wishes to return the favour and, at risk to himself, transports Carmiña to the French border, advising her not to return to Spain.

In France, in her nurse’s uniform, Carmiña helps out with the wounded and manages to avoid being taken to a refugee camp that is more like a prison. She is helped by a woman, Irene, who works for the Post Office. Irene locks her in the store, where unbeknown to Carmiña she sleeps on top of a sack containing her daughters’ letter, and then arranges for her to be smuggled out of the country to Argentina. After the Spanish Civil War ends, the children’s letter makes its way to Madrid, where it is viewed with suspicion because it has come from Russia. The censor, Boniface, had been a postman until he was run over by a car. He understands the letter has been written by an unhappy child and, while the other censor in the office is suspicious, he approves the letter and sends it on to its destination.