In the early hours of the morning, the Soviet Union's early-warning systems detected an incoming missile strike from the United States. Computer readouts suggested several missiles had been launched. The protocol for the Soviet military would have been to retaliate with a nuclear attack of its own.
But duty officer Stanislav Petrov - whose job it was to register apparent enemy missile launches - decided not to report them to his superiors, and instead dismissed them as a false alarm.
This was a breach of his instructions, a dereliction of duty. The safe thing to do would have been to pass the responsibility on, to refer up.
But his decision may have saved the world. ...
"The siren howled, but I just sat there for a few seconds, staring at the big, back-lit, red screen with the word 'launch' on it," he says.
The system was telling him that the level of reliability of that alert was "highest". There could be no doubt. America had launched a missile.
"A minute later the siren went off again. The second missile was launched. Then the third, and the fourth, and the fifth. Computers changed their alerts from 'launch' to 'missile strike'," he says. ...
Although the nature of the alert seemed to be abundantly clear, Mr Petrov had some doubts.
Alongside IT specialists, like him, Soviet Union had other experts, also watching America's missile forces. A group of satellite radar operators told him they had registered no missiles.
But those people were only a support service. The protocol said, very clearly, that the decision had to be based on computer readouts. And that decision rested with him, the duty officer.
But what made him suspicious was just how strong and clear that alert was.
"There were 28 or 29 security levels. After the target was identified, it had to pass all of those 'checkpoints'. I was not quite sure it was possible, under those circumstances," says the retired officer.
Mr Petrov called the duty officer in the Soviet army's headquarters and reported a system malfunction. ...
Now, 30 years on, Mr Petrov thinks the odds were 50-50. He admits he was never absolutely sure that the alert was a false one.
He says he was the only officer in his team who had received a civilian education. "My colleagues were all professional soldiers, they were taught to give and obey orders," he told us.
So, he believes, if somebody else had been on shift, the alarm would have been raised.
--Pavel Aksenov, BBC, on how lucky we are