In the year 2027, the world has plunged into chaos following 18 years of unprecedented human infertility, which has pushed civilization to the brink of complete collapse. With war and global depression rampant, the remnants of society struggle for survival, leading the United Kingdom to become a totalitarian police state. In this grim reality, refugees face horrific fates, often being arrested, deported, or executed in brutal concentration camps, as the government enforces draconian measures to maintain control.
Enter Theo Faron, a once-idealistic activist who has now become a jaded bureaucrat, living in a society where hope seems all but lost. His life takes a dramatic turn when he is captured by the Fishes, a militant group advocating for refugee rights. Among them is his estranged wife, Julian Taylor, who leads this group. The couple's union was shattered after their son tragically died during the 2008 flu pandemic. In an unexpected reunion, Julian offers Theo money to procure transit papers for a young refugee named Kee, who represents a fragile glimmer of hope in a world overshadowed by despair. Out of desperation and the allure of a financial reward, Theo agrees to help Kee and begins to navigate the dangerous waters of this fractured society.
After successfully obtaining the required documents from his cousin, a government minister, Theo prepares to escort Kee. Accompanying them is a band of characters including Luke, a fervent leader of the Fishes, and Miriam, a former midwife. Their journey takes a dark turn when they are ambushed by an armed gang, resulting in the shocking death of Julian. As the chaos unfolds, Luke kills two police officers trying to intervene, forcing the group to hide Julian's body and flee to a pre-arranged safe house.
It is at this safe haven that Kee drops a bombshell: she is pregnant, miraculously becoming the only known expectant mother in a devastated world. Julian's original plan was to take Kee to the Human Project, a covert scientific initiative located in the Azores, dedicated to researching a cure for humanity's infertility crisis. However, tensions rise as Luke seizes power within the Fishes, and suspicions grow when Theo discovers that the very group he thought was helping him orchestrated Julian's demise to elevate Luke's position while plotting against him.
Seeking refuge, Theo, Kee, and Miriam escape to the home of Jasper Palmer, a friend of Theo's and a disillusioned former political cartoonist. Jasper's own life is marred by tragedy, as his wife, Janice, has been rendered catatonic due to her journalistic endeavors, tortured by the oppressive government. Together, they devise a plan to reach the Human Project's ship, Tomorrow, which is set to arrive off the coast of Bexhill, a notorious refugee detention center. Jasper uses his illicit connections to enlist the help of Syd, an immigration officer with whom he shares a business arrangement.
However, their plans are thwarted when the Fishes track down Jasper's location, prompting a hasty escape. In a desperate act, Jasper decides to stay behind, euthanizing Janice and their beloved pet dog with poison before facing the inevitable violence brought upon by Luke and his faction. Meanwhile, Theo, Kee, and Miriam manage to link up with Syd, who assists them in boarding a bus to the refugee camp. The tense situation escalates when Kee's water breaks while on the bus, drawing the attention of a guard who is tasked with selecting individuals for execution. In a moment of bravery, Miriam steps forward, obstructing the guard, while Theo cleverly deceives the guard into believing that Kee has soiled herself.
Once inside Bexhill, Theo and Kee encounter a Romani woman named Marichka, who graciously provides them with shelter for Kee to give birth to a baby girl in the small room she offers. As they settle into their new circumstances, Syd informs them of a brewing conflict, with war erupting between the British military and the desperate refugees. Complications deepen when Theo and Kee learn that their heads carry a bounty, and Syd attempts to apprehend them. With Marichka's assistance, they manage to subdue Syd briefly, leading them to roadside hideouts in hopes of reaching their vessel.
However, the conflict is far from over as the Fishes ambush them again, successfully capturing Kee and her newborn. In the midst of relentless gunfire from British troops, Theo doggedly pursues the Fishes to an apartment building where he confronts Luke, who meets his end in a catastrophic explosion. In a heart-stopping moment, Theo reunites with Kee and the baby amidst the chaos, catching the attention of the soldiers and allowing them a momentary reprieve from fighting, entranced by the sight of new life.
Marichka helps guide them to the rowboat they need to escape, though she selflessly chooses to stay behind as they embark on their journey. As British fighter jets streak overhead conducting airstrikes on Bexhill, Theo and Kee navigate through the fog in their rowboat towards the rendezvous point. In a tender moment of connection, Theo reveals that he was wounded by Luke earlier, and in a poignant exchange, he teaches Kee how to comfort and burp her baby, who she names Dylan in memory of their lost son. As hope flickers in their hearts, Theo's strength wanes, and in a heartbreaking moment, he loses consciousness while the ship Tomorrow approaches. The screen fades to black, but not before the joyful sound of children's laughter echoes, leaving a sense of possibility hanging in the air.
