James and Hilda Bloggs are a retired couple living in a tidy isolated cottage in rural Sussex in southeast England. James frequently travels to London to read the newspapers and keep abreast of the deteriorating international situation; while frequently misunderstanding some specifics, he is fully aware of the growing risk of a nuclear war with the Soviet Union. James is horrified at a radio news report stating that a war may be only three days away, and sets about preparing for the worst as instructed by his government-issued Protect and Survive pamphlets. As Hilda continues her daily routine, and their son Ron dismisses such preparations as pointless (referencing the song “We’ll All Go Together When We Go” by Tom Lehrer), James builds a lean-to shelter inside their home (which he consistently calls the “inner core or refuge” per the pamphlets) and prepares a stock of supplies. He also follows through seemingly strange instructions such as painting his windows with white paint and readying sacks to lie down in when a nuclear strike hits. Despite James’ concerns, he and Hilda are confident they can survive the war, as they did World War II in their childhoods, and that a Soviet defeat will ensue.
Hearing a warning on the radio of a missile being launched James rushes himself and Hilda into their shelter, just escaping injury as distant shock waves rack their home. They remain in the shelter a couple of nights, and when they emerge they find all their utilities, services and communications have been destroyed by the blast. Over the following days, they gradually grow sick from radiation poisoning. Ron and his wife Beryl are not heard from again.
In spite of all this, James and Hilda stoically attempt to carry on, preparing tea and dinners on a camping stove, noting numerous errands they will have to run once the crisis passes, and trying to renew their evaporated water stock with (contaminated) rainwater. James keeps faith that a rescue operation will be launched to help civilians. Apparently oblivious to the dead animals, ruined buildings and scorched, dead vegetation outside their cottage (apart from their own garden), they initially remain optimistic. However, as they take in the debris of their home, prolonged absence of other human company, lack of food and water, growing radiation sickness, and confusion about the events that have taken place, the couple begins to despair.
After a few days, the Bloggs are practically bedridden, and Hilda is despondent when her hair begins to fall out, after vomiting, developing cancerous and painful sores and lesions and experiencing bleeding gums. Either in denial about the extent of the nuclear holocaust, unable to comprehend it, or trying to comfort Hilda, James is still confident that emergency services will eventually arrive, but they never do, as they were also presumably destroyed in the attack. The film ends with the dying James and Hilda getting into paper sacks, crawling back into the shelter, and praying. Jim begins with the Lord’s Prayer, but then switches to the first lines of “The Charge of the Light Brigade”, whose militaristic and ironic undertones distress the dying Hilda, who weakly begs him not to continue. Finally, Jim’s voice mumbles away into silence. The last frames of the film focus on the outside of their shelter, which is seen slowly floating through and into the darkened radiated cloudy sky, then disappears into a brightening sun, and the whole scenery turned into a beautiful blue sky with clouds floating by. At the very end of the closing credits, a Morse code signal taps out “MAD”, mutual assured destruction.