Part I of A Thousand Splendid Suns begins in the early 1970s, when Mariam is a teenager living with her mother, Nana, in a kolba or small hut outside of the city of Herat. We learn that Mariam is the illegitimate child or harami (“bastard”) of Nana and Jalil, a wealthy cinema owner in Herat. Mariam is taught to recite verses from the Koran by Mullah Faizullah, whom she looks up to and admires. Jalil comes to visit Mariam every week, and though Nana tries to convince Mariam that Jalil is embarrassed by her and refuses to consider her a true member of his family, to Mariam, he can do no wrong.
One day, against Jalil’s wishes, Mariam descends the hill into Herat for the very first time in order to see him. She is told he isn’t there, and after spending the entire night sleeping on his stoop, his chauffeur brings her back to the kolba, though not before she has a glimpse of Jalil looking down at her from the window. Upon their return, Mariam sees her mother hanging from a rope. She feels desperately guilty, especially now that she knows Nana was right about Jalil. She loathes him even more once he marries her off to Rasheed, a shoe shop owner in Kabul thirty years her senior.
In Kabul, Mariam is astounded by the cosmopolitan atmosphere, though Rasheed makes her wear a burqa and mainly stay within the home. Rasheed initially shows Mariam around the city and buys her gifts, but after she suffers multiple miscarriages he grows sullen and hostile, yelling at her and beating her.
Part II shifts to the perspective of Laila, who is growing up in Kabul not far from Rasheed and Mariam’s house, but who is getting an education thanks to her progressive father, Babi. But Mammy, her mother, is depressed and unable to take care of Laila because she so misses her two sons, Ahmad and Noor, who have gone to fight with the Mujahideen against the Soviets. Mammy’s depression worsens even more after the two boys are killed. However, Laila has far more happy childhood moments than Mariam, from her walks home from school with her friends Giti and Hasina, to her lessons with Babi and, above all, her friendship with the mischievous Tariq, who lost one leg in a land mine accident when he was five. Tariq and Laila together witness the departure of the Soviets from Afghanistan. Their relationship turns romantic just as the Mujahideen’s infighting begins, and they sleep together for the first time just before Tariq’s family flees for Pakistan. Not long after that, Laila’s family is preparing to leave as well when a rocket hits their home and kills both her parents.
Part III alternates between Mariam’s and Laila’s point of view with each chapter. Rasheed digs Laila out of the rubble of their home, and Mariam slowly nurses him back to health. However, it soon becomes clear that Rasheed’s apparent kindness has hidden his true goal—to take Laila as his second wife. Mariam begs him not to, but Rasheed threatens to turn Laila out onto the streets. Laila agrees to wed Rasheed—she has become pregnant with Tariq’s child, and knows this is the only way to save the baby and herself. Mariam despises Laila, and the two live together in constant tension and low-simmering hostility. Not long afterward, a man named Abdul Sharif comes to the house and says he was in a hospital with Tariq, whose lorry (truck) had been caught in crossfire on the way to Pakistan and who was gravely wounded and, Abdul Sharif says, died.
Rasheed is initially solicitous and adoring of Laila. After Laila gives birth to a baby girl, Aziza, however, he grows once again irritable and even violent, angry it was not a boy. At one point, Laila tries to stop Rasheed from beating Mariam. This small act leads the tensions between the two women to cool, and after drinking several cups of chai together, they start to become close friends and allies rather than adversaries. Laila confides to Mariam that she has been stealing bit by bit from Rasheed and plans to escape to Pakistan in the spring. Together with Aziza, the two of them depart for the Kabul bus station and ask a kind-looking man to pretend he’s their cousin accompanying them out of the city—the Mujahideen prevent women from travelling alone. But the man betrays them, and Laila and Mariam are questioned before being taken back to Rasheed’s, where they are both beaten severely and locked into separate rooms.
The Taliban take control of Afghanistan shortly afterward, and begin to implement strict Shari’a, a strict set of religious laws that prevent women from working and severely restrict their freedom and mobility. Around the same time, Laila realizes she is pregnant with Rasheed’s child. She comes close to aborting the child on her own, but ends up deciding that she cannot accept what the Mujahideen had accepted—that sometimes in war innocent life must be taken. She gives birth to a boy, Zalmai, in a harrowing caesarian at the only women’s hospital still open in Kabul, which no longer has any anesthetic. Zalmai is cheerful and playful, but he has a malicious streak that comes out when he’s with his father, who spoils him while largely ignoring Aziza.
Several years later, during a massive drought, Rasheed loses his business in a fire and the family begins to go hungry. Mariam tries to call Herat to speak with Jalil, but learns that he died back in 1987—not long after he came to Kabul to see Mariam, but she refused to see him. Rasheed forces Laila to send Aziza into an orphanage. He rarely agrees to accompany her and Mariam to see Aziza, though when he doesn’t, Laila leaves on her own and endures frequent beatings by the Taliban for being a woman on the street alone.
One day, Laila, Mariam, and Zalmai are returning from the orphanage when Zalmai calls out that there’s a strange man outside the house. It’s Tariq—it turns out that Rasheed had hired Abdul Sharif to concoct the story of Tariq’s death in order to force Laila to accept her marriage to Rasheed. Instead, Tariq had made it to a refugee camp. There he attempted to make money for his family by transporting coats across the Pakistani border, but the police found drugs inside the coats. He had been imprisoned for seven years before leaving for Murree, Pakistan and saving up money by working at a hotel. Laila tells him about Aziza and they make plans for him to meet her. That night, however, Zalmai tells Rasheed about the strange man Laila was talking to. Rasheed sends him upstairs and begins to beat Laila and Mariam. When Laila hits him back, Rasheed flies on top of her and begins choking her. Mariam, seeing he means to kill her, takes a shovel from the toolshed and breaks it over Rasheed’s head, killing him.
Mariam initially comforts Laila by convincing her they will run away together and lead a quiet peaceful life in a small village somewhere. The next day, however, Mariam tells her that she cannot allow Laila and her family to suffer for Mariam’s actions. She says she could never have hoped for the love and sense of belonging she experienced through her friendship with Laila. Mariam turns herself in to the Taliban. After a brief trial, she is imprisoned and then sent to Ghazi stadium to be executed.
Part IV opens with Laila and Tariq living in Murree and working at a hotel. Though Laila enjoys her life in Pakistan, she knows that Mariam did not sacrifice herself so that she could be a maid in a foreign country. The family returns to Afghanistan, first stopping at Herat. Laila meets Mullah Faizullah’s son, Hamza, and sees where Mariam grew up. Hamza gives her a box that Jalil had left for Mariam, which includes a letter in which Jalil asks for Mariam’s forgiveness and encloses her part of the inheritance—a token arriving too late for Mariam.
The novel closes with Laila working at the same orphanage where she had sent Aziza, teaching and working to renovate the building. She is pregnant with her third child, and knows that if it’s a girl, the baby will be named Mariam.