Mrs. Which responds to Meg's question, saying that her father is "beehindd thee ddarrkness" (5.1).
Meg begins to cry, but Mrs. Whatsit reassuringly enfolds Meg in her wings and wordlessly communicates that there is still hope.
Mrs. Which tells them that they must go behind the shadow, and Mrs. Whatsit says that they will tesser to get there.
Mrs. Whatsit and Mrs. Who attempt to explain a tesseract to Meg: Mrs. Who holds out the edge of her robe in two hands with a straight line of fabric between them (there are pictures in the book to illustrate), and Mrs. Whatsit says to imagine an insect walking from one of Mrs. Who's hands to the other. It would be a long distance to walk – unless Mrs. Who brings her hands together and folds the cloth, and that's a tesseract. (Got it? Meg didn't, and we must confess we don't entirely either.)
Charles Wallace tries: the first dimension is a line, the second dimension is a square, the third dimension is a cube, the fourth dimension is...time, sort of, and the fifth dimension is time squared, which is a shortcut through all the other dimensions.
Meg almost gets it, enough that we can all move on.
Calvin takes Meg's hand and they set out once more through nothingness, a.k.a. the Fifth Dimension.
Meg expects the nothingness this time so that's not so bad, but she doesn't expect to feel at one point like she's being squished flat, unable to breathe or even to have her heart beat.
When they finally arrive wherever it is they're going, Mrs. Which apologizes: she tried to land them on a two-dimensional planet, forgetting that humans bodies don't work so well when they're flattened.
Meg is concerned that her mother is going to be worried about them, but Mrs. Whatsit (now back in her humanish form) explains that they took a tesser in time as well in space, so they'll return (if they do return) before they left. Which means no time will have passed.
Meg finally notices the planet they are on, which is wholly unremarkable, with gray air that blocks out everything at any distance.
Mrs. Whatsit leads them to a cave, which she explains is the home of the Happy Medium.
Mrs. Which tells Mrs. Whatsit that just because she's young doesn't mean it's OK for her to be such a chatterbox.
Meg asks Mrs. Whatsit how old she is and she replies that she is 2,379,152,497 years, 8 months, and 3 days old – in Earth years, of course.
They come upon the Happy Medium, who looks like a very cheerful fortune teller, with a turban and a crystal ball and everything.
Mrs. Whatsit makes the introductions and asks the Medium to show the children their home planet in her crystal ball.
The Medium becomes somewhat less Happy at this prospect, and it's soon clear why: the same Black Thing that the children saw in the sky is also surrounding Earth.
Mrs. Whatsit explains that it's been there for quite some time, and Mrs. Which says that it is "Eevill [...] thee Ppowers of Ddarrkknesss" (5.109).
Meg asks what they're going to do and Mrs. Which responds with her equivalent of "we're going to kick some serious butt!"
Mrs. Whatsit adds that everyone, from the smallest microbe to the largest galaxy, is joining in the fight.
At the prompting of Mrs. Whatsit and Mrs. Who, the children name off some shadow-boxing ninjas from their own planet: Jesus, da Vinci, Michelangelo, Shakespeare, Bach, Madame Curie...
Meg's still got her dad on the brain, and Mrs. Whatsit explains that he's on a planet that has succumbed to the Black Thing, so it's going to be trouble.
The Happy Medium, now downright sad, tells the children to watch her crystal ball.