In an unspecified nation, the nameless officer X-127 is taken to an ultra-secret installation called Level 7. He has been chosen to be one of four "Push Button Command" officers, whose task is to fire the nation's arsenal of missiles in the event of war.
Level 7 is 4,400 feet down, and there is no way out. 500 people are there forever, with food and water and power to last 1000 years. They aren't told of the one-way nature of the assignment until they are actually in Level 7, however, and some of them don't take well to their situation.
PBX Officer X-127 decides to write a diary of his experiences, and the book takes the shape of this diary.
Little things begin to cause stress; X-127 is obsessed with the fact that he will never see sunlight again, and others become depressed when the music tape played over the omnipresent Loudspeakers turns out to be limited to a 12 day loop.
The Loudspeaker, source of all information and entertainment, announces that marriages will be allowed.
After some self-debate, X-127 marries P-867, a psychologist.
He doesn't love her but thinks being married to her will help him cope.
X-127 muses on the social stratification of Level 7 and those above, deciding that they have found "perfect democracy."
At last, the Loudspeaker orders the PBX officers to carry out their function: a keyboard of buttons labelled A, B, C and 1-4 are set to fire missiles of varying range and yield, starting with A1 and ending with massive dirty bombs launched by buttons 4.
Unstable officer X-117 freezes up as the war is carried out, and must be replaced.
All the buttons are pressed, and in 2 hours 58 minutes the world is eradicated.
Shelters at Level 1 and 2 are mostly destroyed, the rest dying of radiation soon after.
Overcome with guilt, X-117 kills himself.
A couple from Level 3 go out to report on conditions, and are dead within 5 days, finding nothing but ash and darkness.
Now, radio contact with The Enemy's shelter continues the war, as both sides claim victory and trade tirades against each other, with neutral nations blaming both.
However, something is wrong. People begin dying on the middle levels. No one knows how or why, but radiation is getting to the supposedly safe havens.
The Enemy radio falls silent, presumably dead.
At last it is discovered that the reactor which fuels Level 7 has leaked, and that the whole population is doomed despite all the precautions.
X-127 is the last human being on Earth, as he writes, "a distinction amid extinction."
His last words are incoherent scrawl as he slowly dies of radiation poisoning, leaving Level 7 itself as the last funtioning "life" on Earth.
This report prepared by tiepilot912