Mr. Murry's absence is where we begin, and it's the situation that all the central characters want to fix.
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.
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.
This is the moment Meg (and we) have been waiting for...everything's going to be all right now, right? Right? Oh. Dang.
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?
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.)
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.