You fight the Black Thing with love.
The Black Thing is a manifestation of evil. It overshadows Earth and other planets in the universe, and it currently has Meg’s father captive on the planet Camazotz in the form of IT. IT is a bodiless brain who controls the entire planet, where everyone is in lockstep and lives in fear of not conforming. When Mrs. Who, Mrs. Whatsit, and Mrs. Which take the children there, Charles Wallace falls under IT’s spell, and now Meg has a problem. Her father and her brother both have been taken captive—one literally, and one mentally.
Meg is able to get her father free using a trick (Mrs. Who’s glasses) and her knowledge of science. She uses her unique way of looking at the world and her love for her father. She compares it to a rice curtain at a beach house.
At first Meg had flinched each time she came up to the curtain; but gradually she got used to it and would go running right through, leaving the long strands of rice swinging behind her. Perhaps the atoms of these walls were arranged in somewhat the same fashion. (Ch. 9)
Meg is able to rescue her father, but has to tesseract out of there without Charles Wallace. She feels terrible about leaving him trapped with the evil IT, but she has no choice. He is under IT’s spell, and the spell of the Black Thing. Meg herself gets too close, and gets terribly ill.
In the end, Meg only gets her brother back from IT and the Black Thing through love. She shows him that she loves him, and is able to get through to him by reminding him that he loves her. No one else would have been able to accomplish this but her.
Her own Charles Wallace, the real Charles Wallace, the child for whom she had come back to Camazotz, to IT, the baby who was so much more than she was, and who was yet so utterly vulnerable.
She could love Charles Wallace. (Ch. 12)
Meg is able to get her brother back, and she and her father return home. The entire family returns home. Fighting the Black Thing is an ongoing activity. He is never really defeated, as Aunt Beast reminds Meg. You can’t let your guard down around evil.
"Oh, yes," Aunt Beast replied. "In doing that we can never relax. We are the called according to His purpose, and whom He calls, them He also justifies. Of course we have help, and without help it would be much more difficult." (Ch. 11)
However, Meg has done what she can for her family. Her family is whole again, and she is back with her father again. Her brother is back again. She has learned something valuable about herself. She has learned that she does have worth, and she is strong.
One of the main lessons of this book is that every person is unique, and what makes us unique can be a strength and not a fault. Meg is a strong and talented young lady, but she considered these strengths her weaknesses before she took this journey. It was these very strengths that she needed in order to help her family, fight evil, and save the universe just a little bit too.