The main topic or theme of this novel is that love is stronger than hate.

In the novel, Meg, Charles Wallace, and Calvin travel to a planet called Camazotz, which has been taken over by the power of evil. Everyone on the planet has been dehumanized, their minds, bodies, and souls controlled by a computer called IT. The children have to free their father, imprisoned by IT. They manage to do so but have to leave Charles Wallace behind. He has been taken over by IT. Meg has to travel by herself back to Camazotz to save him. She does this, finding out that the weapon evil can't fight is the intense love she has for her little brother. Love defeats evil, and Meg can bring Charles Wallace home to earth.

The novel expresses and illustrates the central Christian theological belief that love is the most powerful force in the universe. Meg grows from adolescent awkwardness and anger to a more self-assured maturity as she realizes this truth about love and also recognizes her own power to be a force for good in the universe. The novel illustrates as well the importance of good people and forces binding together in a community to fight evil.