A Swiftly Tilting Planet
Madeleine L'Engle
 The book opens on Thanksgiving evening, about 10 years after the events of A Wind in the Door. Meg is now married to Calvin and is expecting their first child. Calvin has become a scientist and is in Britain at a conference. The Murry family is joined for Thanksgiving dinner by an unusual guest—Meg's very antisocial mother-in-law, Mrs. O'Keefe. When they receive the news of impending nuclear war caused by the dictator "Mad Dog Branzillo", Mrs. O'Keefe lays a charge on Charles Wallace &mdash; to prevent the disaster. She teaches him "Patrick's Rune", a rhyming prayer of protection that has been passed down to her from her Irish grandmother. Charles Wallace goes for a walk to the star-watching rock, a family haunt, and begins to recite Patrick's Rune. His recitation summons a flying unicorn from the heavens who introduces himself as Gaudior. The winged unicorn explains to Charles Wallace that he must prevent nuclear war by traveling through time and telepathically merging with people who lived in the locale of the star-watching rock at points in the past. By doing so he may change pivotal situations, "might-have-beens", in which things might have turned out better than they did. Though unsure how events in the distant past near his home can affect a South American dictator, Charles Wallace agrees. They are threatened along the way by the Echthroi, the antagonists introduced in A Wind in the Door. The Echthroi are evil beings whose goal is the total destruction of the universe. As Charles Wallace tries to make the might-have-beens turn out for good, the Echthroi are fighting to make them turn out for evil. Gaudior and Charles Wallace's travels bring them to: Harcels, a Native American boy at least 1,000 years in the past; Madoc Gywnedd of Wales, a pre-Columbian trans-oceanic traveler; Brandon Llawcae, a Welsh settler in puritan times; Matthew Maddox, a writer during the American Civil War who wrote a novel about the legend of Madoc Gwynedd; and Mrs. O'Keefe's brother Chuck Maddox, during their childhood. Meg connects with Charles Wallace from home through "kything", the telepathic communication she learned in A Wind in the Door. Eventually, a connection arises between the star-watching rock and Mad Dog Branzillo. In the 12th century, two Welsh princes named Madoc and Gwydyr travelled to North America to escape the in-fighting for their father's throne. But once there, Gwydyr turned against Madoc and tried to conquer their new home. Madoc defeated Gwydyr in combat, and Gwydyr left to South America. Both men married into the local Native American populations and became part of the local folklore. Many generations later in the 1860s, a descendant of Madoc marries a descendant of Gwydyr, and Mad Dog Branzillo is their descendant. However, thanks to Charles Wallace's changing of "might-have-beens" in the history of Branzillo's ancestors, things turn out differently. Distant descendants of Madoc marry, reuniting Madoc's line and resulting in a peaceful man being born instead, and the threat of nuclear war is dissolved.