A moment ago, Meg was standing with Calvin and Charles in the backyard of their house. Now, all of sudden, she is whipped into nothingness. All is dark, and she can't feel her body. Then feeling in her body slowly begins to return, and she hears but cannot see Calvin and Charles. They all seem to be trying to push through this barrier back into reality, and Meg is the last one to break through.
Meg is plunged into the bizarre feeling of a tesseract, something beyond her mental comprehension but which is nonetheless a physical reality for her. It is interesting that it is hardest for Meg to break through, as she is the one who always wants easy answers.
Senses returned to them, they find themselves standing in a beautiful field in springtime (it was very much autumn in the Murry's village) with a tall mountain in the background. Mrs. Whatsit and Mrs. Who materialize nearby, and Mrs. Which appears as a shimmery witch, which Mrs. Whatsit and Mrs. Who find hilarious. The children are rather cross as there has been no explanation about what is going on, so Mrs. Whatsit explains the situation a bit: the life of Mr. Murry, and of much more, is at stake. They're currently standing on a planet outside Earth's galaxy, and they got there by a tesseract, which is a way to travel instantaneously through space and time.
Mrs. Which appearing as a "witch" was obviously her idea of a joke and a play on words, a sign of how human language is just a plaything to the Mrs. W's. Her appearance as a witch is not representative of the being she actually is.
At this point, at Mrs. Which's command, Mrs. Whatsit shape-shifts into her real form. She is not a funny old woman, but a magnificent winged horse more beautiful and noble than can be imagined. Calvin falls to his knees, but Mrs. Whatsit sternly commands him to rise. The three children clamber onto her back, and they take flight.
Here, the deceiving nature of outward appearances really hits home with the children: Mrs. Whatsit, the clumsy and ridiculously-dressed old woman, is actually the most beautiful creature one could possibly imagine.
As they fly over a group of creatures just like Mrs. Whatsit, they hear them making a beautiful music whose song Mrs. Whatsit roughly translates into their human language as a passage from Isaiah: "Sing unto the Lord a new song…" The children feel a great peace and joy.
Though they are no longer on Earth and though neither the children nor the Mrs. W's have made any mention of being the Christian, the indescribably beautiful song of the creatures on this planet translates best into a Biblical verse from Isaiah. While this does not suggest that the Mrs. W's are Christian, it does suggest that the Bible expresses truths in ways that are universal.
As they fly higher, the children are given oxygen-omitting flowers so they can breathe in the thinning atmosphere. Mrs. Whatsit reaches the top of the mountain, and the children disembark to gaze at the stars above, whose light seems to be dimmed by a dark, terrifying shadow. When they return to the ground, Meg asks Mrs. Which if that Black Thing is what her father is fighting.
Although in the case of Meg, Calvin, Charles, and Mrs. Whatsit, appearances are deceiving, the children are shown the Black Thing in a very visual way in order for them to understand it. Still, since it is only a shadow, it is difficult to see, and in that sense is immaterial.