Jeanne Marwan, a graduate student in mathematics, explains the Collatz conjecture to a class of undergraduates, where every positive integer leads to a sequence that ultimately ends in the number two reducing to the number one.
Following the death of Jeanne’s mother Nawal, an Arab immigrant in Canada, Jeanne and her twin brother Simon meet with French Canadian notary Jean Lebel, their mother’s employer and family friend. Nawal’s will makes reference to not keeping a promise, denying her a proper gravestone and casket, unless Jeanne and Simon track down their mysterious brother, whose existence they were previously unaware of, and their father, whom they believed was dead. Jeanne accepts; Simon, on the other hand, seemingly having had a more difficult relationship with Nawal and her apparently unusual personality, is reluctant to join Jeanne on this pursuit.
A series of flashbacks reveal Nawal came from a Christian Arab family in an unnamed Middle Eastern country, and that she fell in love with a refugee named Wahab, resulting in her pregnancy. Her family murders her lover and nearly shoots her as an honour killing, but her grandmother spares her, making her promise to leave the village after the birth of her baby and start a new life in the fictional city of Daresh. The grandmother tattoos the back of the baby’s heel and sends him to an orphanage.
While Nawal is at university in Daresh, a civil war and war crimes break out with Nawal opposing the war on human rights grounds. Her son’s orphanage in Kfar Khout is destroyed by a Muslim militant, Chamseddine, who converts him into an Islamic child soldier. After barely escaping the massacre of a bus full of Muslim refugees by Christian Nationalists, Nawal narrowly manages to join the Muslim fighters, and eventually shoots a nationalist leader. She is imprisoned in Kfar Ryat and raped by torturer Abou Tareq, consequently giving birth to the twins.
After travelling to her mother’s native country, Jeanne gradually uncovers this past, and persuades Simon to join her. With help from Lebel, they learn their brother’s name is Nihad of May and track down Chamseddine. Simon meets with him personally, and he reveals the war-mad Nihad was captured by the nationalists, turned by them, trained as a torturer, and then sent to Kfar Ryat, where he took the name Abou Tareq, making him both the twins’ half-brother and father; as such, the two people they were seeking reduce to one. Nihad had immigrated to Canada and Nawal only learned his true identity after recognizing the tattoo on the back of his heel at a Canadian swimming pool. The shock of learning the truth caused Nawal to suffer a stroke which led to her decline and untimely death at age sixty.
The twins find Nihad in Canada and deliver Nawal’s letters to him without speaking to him. He opens both of them; the first letter addresses him as the twins’ father, and is filled with contempt, as Nihad is written to by Nawal as her rapist. The second letter addresses him as the twins’ brother, and is instead written with caring words, saying that he, as Nawal’s son, is deserving of love.
Nawal gets her gravestone in the aftermath of the letters being sent, which Nihad visits.