Where and why do Calvin, Charles Wallace, and Meg go with Mrs. Whatsit, Mrs. Who, and Mrs. Which in the book A Wrinkle in Time?

The
children travel to three destinations with the mysterious women. First, before Meg and Calvin
know what is happening, they find themselves whisked away, along with Charles Wallace, to the
planet Uriel. During the trip, Meg loses all sensation as she floats through complete
nothingness. She takes the longest to recover, and Charles is afraid that Mrs. Whatsit left her
behind. Then, the three women begin to hint that the children are traveling to help Meg's
father.

Mrs. Whatsit assumes her true form, a hybrid horse-human shape (yet
"not like a Greek centaur. Not in the least"). Meg, Calvin, and Charles board Mrs.
Whatsit, who takes them into the sky. There, they encounter the Thing for the first time. After
the children and Mrs. Whatsit return to Mrs. Who and Mrs. Which, the entire group then travels
(or rather, tessers) to Orion's Belt to visit the Happy Medium. The Medium shows the children
their planet, where the Thing has been spreading for years. Then, at the children's request, she
reveals their mothers. Meg, seeing her mother's despair, grows impatient to find her
father.

Mrs. Who, Mrs. Which, and Mrs. Whatsit transport the children to
Camazotz, where Meg's father is. After a few commands and impartations, the women depart. Meg,
Charles, and Calvin must work together to help Mr. Murry and to bring him
home.