The narrative opens by detailing the nuclear deterrence strategy of Great Britain, which relied heavily on the threat of total devastation delivered via the Royal Air Force's nuclear-armed V bombers. Because of the vast number of V bomber bases—many of which would be dispersed across the countryside during a crisis—combined with the presence of major civilian population centers, the film posits that Britain possessed a higher density of potential nuclear targets by area than any other nation on Earth.
The geopolitical tension reaches a breaking point on September 15, when American forces stationed in South Vietnam are granted authorization to utilize tactical nuclear weapons to repel a Chinese invasion. This escalation prompts a severe response from the Soviet Union and East Germany, who threaten a full-scale invasion of West Berlin unless the United States reverses its course. By the following day, the British government officially declares a state of emergency, shifting the daily administration of the country to a network of regional commissioners.
The immediate priority for these newly formed emergency committees is the mass evacuation of the most vulnerable populations—specifically children, mothers, and the infirm—who are relocated to designated safe zones, including the county of Kent. This process is far from voluntary; homeowners are forced to accommodate evacuees under the threat of imprisonment, and the government begins the requisitioning of any unoccupied properties. To manage the dwindling resources, rationing is strictly implemented. The public is issued instructional booklets on nuclear survival, and emergency sirens are tested frequently. These sirens are estimated to provide a mere three minutes of warning before an impact, and potentially less than thirty seconds if the attack originates from a submarine. Despite the looming threat, there are no government-provided shelters, and private attempts to build fallout shelters are crippled by a severe shortage of construction materials.
On September 18, the inevitable occurs: the Soviet Union and East Germany invade West Berlin. NATO attempts a counterattack, but the effort is quickly overwhelmed, leading to the American deployment of tactical nuclear weapons. In a swift and deadly reaction, the Soviets launch their own nuclear arsenal at strategic targets, driven by the knowledge that their liquid-fuelled missiles are highly vulnerable to a first strike.
In Kent, a one-megaton warhead intended for RAF Manston overshoots its target and detonates in an air burst six miles from Canterbury. The resulting blast wave creates intense, searing heat that strikes the city with merciless precision. In one harrowing scene, a defense worker and a young boy in a yard are hit by the heat wave, which is so extreme that it causes their eyeballs to melt. Inside a nearby home, furniture spontaneously ignites, sending the occupants into a blind panic as they struggle to extinguish the flames, only for the entire structure to be leveled twelve seconds later by the incoming shockwave. Elsewhere, a boy suffers from flash blindness after looking directly at an explosion twenty-seven miles away; his father desperately carries him inside to hide under a table as the house trembles from successive distant detonations. In Rochester, an airburst triggers a catastrophic firestorm that consumes the town in flames. Simultaneously, British V bombers penetrate Soviet airspace to deliver a reciprocal level of devastation.
The aftermath leaves Kent's emergency services completely paralyzed. Surviving doctors are overwhelmed, with each professional tasked with treating at least 350 casualties. The most critically injured are often left to die in isolation or are granted "mercy killings" via police gunfire. Survivors are plagued by severe PTSD, and radiation sickness becomes rampant across the region. To manage the dead, bodies are burned in mass pyres; to prevent grieving relatives from interfering with these grim tasks, affected areas are sealed off by armed police. With essential utilities and supplies non-existent, society begins to collapse.
The remaining food supplies are reserved exclusively for those maintaining "law and order," which sparks violent riots among the starving populace. These clashes evolve into armed skirmishes between desperate civilians and the authorities, resulting in civilians seizing food warehouses and trucks carrying weaponry. The state responds with brutality; individuals accused of obstructing government officers or causing civil unrest are executed by police firing squads. Among those executed is the father of the blinded boy mentioned previously. As the food shortage persists, the population begins to suffer from scurvy due to a total lack of vitamin C.
The film concludes on Christmas Day at a refugee facility in Dover. Orphaned children, when asked about their future aspirations, either remain silent or state that they "don't want to be nothing." The tragedy is further emphasized by the plight of a child with a chronic, leukemia-like illness who is estimated to have only seven bedridden years left to live, and a pregnant woman exposed to radiation who fears her child will be stillborn. The film ends with a sobering observation that the real-world press says nothing about the actual dangers of nuclear weaponry, while a distorted, damaged recording of the hymn Silent Night plays over the closing credits.
