The first half of the programme looks at the physical effects of a single medium-sized nuclear warhead on a large city. How effectively would various types of shelter protect us from these effects? Two couples experimentally live in fall-out shelters for ten days each. The implications of the film are then discussed by medical, scientific and civil defence specialists. Would cities be likely targets in a real nuclear attack? What kind of resources would be available to deal with the aftermath of such a calamity? And what could we do - practically - to diminish the effect if it ever were to happen?