A Swiftly Tilting Planet presents Charles Wallace with a quest to save the world from Maddog Branzillo, who would start nuclear war. He is given only an ancient rune as clue by a mysterious Mrs. O’Keefe. Charles Wallace goes out to the star-watching rock to contemplate and, after saying the rune, is greeted by Gaudior, a unicorn who takes him back and forth through time on the wind as he attempts to discover what in history caused the nuclear threat to appear in the present.

L’Engle explores the idea of the butterfly effect—the idea that one decision or flap of a butterfly’s wing can affect the present or the future—in this final novel of the original trilogy. She gives Charles Wallace the ability to go within people as he travels and to live through their eyes, hopefully changing for the better the outcome of his present. Charles Wallace’s mission is to find the might-have-been whom he can change, therefore preventing the present threat from ever having happened.

Meg is married and pregnant with Calvin’s baby, and her function in this novel is to support Charles Wallace and kythe with him her love and the information that she finds out about the history he is experiencing. The plot moves between her experiences of what he sees and his conversations with Gaudior.

This novel allows Charles Wallace to develop his talents and to learn patience and love for all the characters from history whom he inhabits. By letting go and being them, he is taken to the right place and is able to change the course of history, with Meg’s support and love to back him. The power of love is present here in two aspects: the love that Meg gives Charles Wallace, which sustains him, and the love that Charles Wallace develops for the people he embodies. This selfless love is what allows him to make the right decision at the right time when he is in the body of the correct person. This supernatural out-of-body experience is the most extreme form of science fiction and fantasy in the series, yet it allows Charles Wallace to understand and love Mrs. O’Keefe, the bitter old mother-in-law of Meg who renounced love years ago and rediscovers it through Charles Wallace.