In A Swiftly Tilting Planet, Charles Wallace Murry, an advanced and perceptive child in A Wrinkle in Time and A Wind in the Door, has grown into an adolescent. His intelligence and remarkable goodness carry him through an adventure in time to save the world from nuclear disaster threatened by Mad Dog Branzillo, the dictator of the fictional South American country of Vespugia. In order to change the outcome of the present, Charles Wallace must change the past, in a series of "might-have-beens", events which are turning points fought over by the powers of good and evil. The book's title is an allusion to the poem, "Morning Song of Senlin", by Conrad Aiken.
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 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 1000 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 Gywnedd; 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 1100s, 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.
At Tara in this fateful hour,
I place all heaven with its power.
And the sun with its brightness
And the snow with its whiteness
And the fire with all the strength it hath
And the lightning with its rapid wrath
And the winds with their swiftness along their path
And the sea with its deepness
And the rocks with their steepness
And the Earth with its starkness
All these I place
By Gods almighty help and grace
Between myself and the powers of darkness!
The verse given as Patrick's Rune is L'Engle's adaptation of an authentic medieval prayer, "Saint Patrick's Breastplate", which in turn is a variation on the Lorica of Saint Patrick. L'Engle's rune invokes the same natural phenomena (sun, moon, lightning, rocks, etc.) as the fourth verse of the hymn "Saint Patrick's Breastplate", attributed to St. Patrick, translated by Cecil Frances Alexander, according to the hymnal used by the Episcopal Church, of which L'Engle was a member.