Half of a Yellow Sun opens on the historic day of Nigerian Independence on October 1, 1960, and trails the lives of its characters through the tumultuous end of the Nigerian Civil War in 1970. The film artfully weaves together historical archival footage, showcasing the dramatic political landscape of Nigeria during this period.
The story follows twin sisters Olanna and Kainene, who, after completing their studies in the United Kingdom and the United States, return to their homeland. They come from a prominent family, with their father being the Igbo Chief Ozobia (played by Zack Orji), a wealthy businessman with significant holdings in Port Harcourt. Olanna, spurning the advances of Finance Minister Festus Okotie-Eboh, chooses instead to live with her passionate lover, the revolutionary professor Odenigbo (portrayed by Chiwetel Ejiofor), who teaches at a university in Nsukka. Meanwhile, Kainene ventures into the business world, taking control of the family assets and eventually falling in love with Richard Churchill (Joseph Mawle), a writer from England.
At Nsukka University, Olanna takes up a sociology lecturing position, where she forms a friendship with Odenigbo’s houseboy, Ugwu (John Boyega). However, not all relationships flourish; Olanna faces distrust from Odenigbo's mother, known as "Mama" (Onyeka Onwenu), who sees Olanna as an outsider and even a witch. Determined to disrupt their relationship, "Mama" employs manipulation and alcohol to sway Odenigbo, causing strife as she arranges a liaison between her servant Amala (Susan Wokoma) and her son. Facing emotional turmoil, Olanna considers leaving, but is encouraged by her Aunt Ifeka (Gloria Young) to stay and work through the challenges.
After a brief affair with Richard, Olanna and Odenigbo reconcile and take on the responsibility of raising Amala’s infant, named Chiamaka but affectionately called "Baby." However, as tensions rise leading to civil war, Richard returns to London but is soon entangled in the chaos, witnessing horrific violence against the Igbo people. Olanna, too, struggles for survival during a race riot and subsequently evacuates her family to Abba in Biafra. Ultimately, she decides to stay in Nigeria and commit to Odenigbo by marrying him.
When Biafra proclaims independence, Richard re-emerges to assist Kainene, who has taken to wartime profiteering. As violence escalates, Olanna and her family find refuge in Umuahia but narrowly escape death during a bombing raid at their wedding. The conflict displaces them further, culminating in a refugee camp, where Olanna reunites with Kainene, who is now surprisingly devoted to aiding the community.
As resources dwindle, Kainene bravely ventures into war-torn Nigerian territory to barter for supplies despite Odenigbo’s protests. Days turn into despair as she remains missing; meanwhile, Olanna and Richard strive to locate her but to no avail. They take solace in the return of Ugwu, who survived the brutal war. Following Biafra's defeat, Richard persists in his search for Kainene, while Olanna, Odenigbo, Ugwu, and "Baby" focus on rebuilding their lives.
In the end, a postscript reveals the haunting truth that Kainene was never found while Richard relocated to Nsukka. Olanna and Odenigbo maintain their marriage for nearly fifty years, and Ugwu blossoms into a writer, while their daughter, Chiamaka (also known as "Baby"), realizes her dream of becoming a medical doctor.
