Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Initial Situation
Mr. Murry is missing
Mr. Murry's absence is where we begin, and it's the situation that all the central characters want to fix.
Conflict
Meg, Charles Wallace, and Calvin finally make it to the planet where Mr. Murry is, but things are not going well.
There's plenty of conflict on Camazotz: between the kids and the man with the red eyes; between Meg, Calvin, and Charles Wallace, as to what they should do; and within the characters, as they struggle to make tough decisions.
Complication
Charles Wallace intentionally submits to IT, thinking he will be able to lean the whereabouts of his father and then escape the mind-control.
Doesn't that child ever listen? Despite being warned by the Mrs. Ws to beware his own smarty-pantsness, Charles thinks he's stronger than he is and gives in to IT, putting Meg and Calvin in a bad spot.
Climax
Meg finds her father.
This is the moment Meg (and we) have been waiting for...everything's going to be all right now, right? Right? Oh. Dang.
Suspense
Finding Dad does not, in fact, solve all Meg's problems.
Meg rescues Mr. Murry, but now there's Charles Wallace to worry about...will they be able to extract him from IT's brainy clutches?
Denouement
Meg rescues Charles Wallace.
Mirroring the climax, Meg returns to Camazotz...and this time it's personal. (OK, it was personal the last time too, but this time Meg takes control in a way she didn't before.)
Conclusion
The Murrys and Calvin, return to Earth for the world's longest group hug.
The situation at the beginning of the book, and the complications along the way, are resolved. Mr. Murry is reunited with his family and everything (it seems) is back to normal.