On Thanksgiving Day, recently married Meg Murry O'Keefe is visiting her family when her father receives a call from the president. A power-hungry South American dictator called "Mad Dog" Branzillo is threatening nuclear war. The news prompts Meg's strange mother-in-law, who shares Branzillo's ancestry, to recite an ancient family rune that causes strange weather disturbances.
It's up to Meg's fifteen-year-old brother, Charles Wallace, to use the rune as he journeys through time, searching for the "Might Have Beens" that led to this crisis. With a unicorn named Gaudior as his guide, Charles Wallace travels in and out of five of Branzillo's ancestors, from prehistoric time tothe 1860s, encountering tensions between two feuding brothers in each generation.
Helped by Meg, who wordlessly links her thoughts with his through kything, Charles Wallace changes the pattern of history and saves the world from destruction.