The climax of the story happens on the dark planet of Camazotz, where Meg and Charles Wallace's father is imprisoned. If I had to identify the most important four, it would be these:
1) Charles Wallace attempts to fight The Man with the Red Eyes and fails. He becomes hypnotized by the Man and acts coldly and dismissively towards the others.
2) Charles Wallace and the Man take Calvin and Meg to see Meg and Charles' father, who has been locked up in a dark room. Using Mrs. Who's spectacles, Meg helps him escape.
3) Charles, still hypnotized, despite Calvin's attempt at shaking him out of it by reciting lines from The Tempest, insists that they all see IT. IT turns out to be huge brain, which tries to pull them into its pulsating rhythm. Meg is slipping when Mr. Murry tessers with her and Calvin in tow.
Interlude! Off of Camazotz, Meg is furious with her father for leaving Charles behind. Everyone is recovering, because Mr. Murry is not super at tessering. Meg realizes that only she can save Charles and returns to Camazotz on her own.
4) Back on the dark planet, Meg confronts her little brother, having discovered the gift she has that IT does not: love. She declares her love for her little brother and they tesser back to their home on Earth.