A Swiftly Tilting Planet (Time Quintet, Bk. 3)

Meg Murry O'Keefe and her family are just sitting down to Thanksgiving dinner when her father gets a phone call from the White House about a madman's threat of nuclear war. Only an old Irish rune seems to hold a clue to averting worldwide disaster, and when Meg's brother Charles Wallace, now fifteen, recites it, a radiant white beast - the unicorn Gaudior - appears to join him on his quest. But there are only twenty-four hours in which to stop tragedy from occurring. Can Charles Wallace, with the help of Gaudior and Meg, possibly succeed?