This great film is really two films in one. The first, is a family drama about Stanislav Petrov looking back on his life and, finally, healing some of the rifts in his family.
The second, which actually takes up the lesser part of the film is the horrifying real-life story of how Russia nearly launched a nuclear attack on the US in the 1980s after faulty equipment indicated missile launches by the Americans.
Petrov was the officer in command when it happened and decided not to believe what his colleagues and equipment was telling him. That one decision saved millions, even billions, of lives and is recreated brilliantly in this movie with huge tension even though we know, of course, what the end result was.
If you like this, you might also be interested in Fail Safe.
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