This story is about a crusading scientist out to stop nuclear testing who is motivated by scientific fact, conscience and faith. Dr. Alex Carmody (Martin Sheen) is a physicist who becomes convinced that if the U.S. and the U.S.S.R. continue to test nuclear weapons, eventually one of these tests will set off an atomic chain reaction that will obliterate the world. Carmody travels to Portugal where he tries talks to one of the women who in 1911 saw a vision of the Virgin Mary that spoke to her and her two companions about the future. Carmody is certain that the Virgin Mary predicted the very chain reaction he and his co-worker Dr. Kenneth Parrish (Peter Firth) envision. Failing in his attempt to talk to the woman, Carmody then travels to Paris and elsewhere, warning Soviet and American officials that the tests they are planning should be cancelled before they become their last.