Well, keep in mind that the climax is a very important part of the plot (in that it is considered a very significant plot element). The climax of a story is found at the end of the rising action but before the falling action. The climax constitutes the absolute highest point of the tension. Although the climax of a story can sometimes be debatable, much of the time it is obvious. In the case of A Swiftly Tilting Planet, the climax of the story is fairly simple: it is the moment when Charles Wallace is on the verge of death.
The climax of this particular book has more to do with a character than it does with the plot itself. The important character here is Charles Wallace who runs as one of the main characters in L’Engle’s book series (of which this is the third installment). Through the course of the book (and mostly encompassing the rising action), Charles and his sister, Meg, are trying to figure out the identity of a dictator in Latin American nicknamed “Mad Dog” Branzillo who, if not stopped, will begin a nuclear war that will end the world. It turns out that Branzillo’s ancestors are incredibly important. It is in “entering” one of those ancestors through a mysterious process that Charles Wallace can perform, that the climax happens.
The minor character of Matthew is handicapped from a horse riding accident in the 1800s and, when Charles Wallace “enters” this character, he is close to death. After learning the truth behind the dictator’s identity, Matthew has a huge coughing fit when Charles “exits” the body and returns to the star watching rock to safety. The group learns from Gaudior (a unicorn who accompanies Charles Wallace) that Matthew was on the point of death when Charles left the body. Charles suffers these effects right here on the star watching rock and almost dies. Meg demands that Mrs. O’Keefe recite a special rune and, only when Meg joins in with her, Charles Wallace begins to come back to life (which happens through light slowly returning and the pain of the handicap disappearing).
After Charles Wallace comes back to life after his strange experience inside Matthew, everything else in the story is falling action and resolution. It is here they learn more about the specifics of saving the world from Mad Dog, and how Mrs. O’Keefe was instrumental in standing in between Meg and the dark powers. Meg and Charles and family have saved the world again.