In response to her questioning, Mrs. Which informs Meg that her father is trapped behind the darkness. Mrs. Whatsit assures her that they are traveling to help him. She explains that they travel by tessering, which involves taking shortcuts through time and space. Seeing that Meg remains confused, Charles Wallace explains that tessering is travel in the fifth dimension: the first dimension is a line; the second is a square; the third is a cube; the fourth is Einstein's concept of time; and the fifth is a tesseract. By adding the tesseract to the other four dimensions, they travel in such a way that the shortest distance between two points is not a straight line. Although Meg does not completely understand, she contents herself with this explanation.
A gust of wind blows the children up, and, as their bodies dissolve beneath them, Meg and the others find themselves tessering. Suddenly, Meg feels herself stopping; her lungs flatten under a tremendous pressure, and she hears a voice saying that the travelers cannot stop now because they are on a two-dimensional planet. Mrs. Which apologizes to the children for her mistake, noting that she is not used to thinking in a corporeal way; she forgets that human bodies cannot exist in two dimensions.
Mrs. Whatsit explains that they are traveling to a foggy gray planet in the belt of the constellation Orion. Meg expresses concern that her mother will be worried about them back on Earth, but Mrs. Whatsit assures her that they have taken a time wrinkle as well as a space wrinkle; they will arrive back home five minutes before they ever left.
Arriving on the foggy planet, the group enters a cave where Mrs. Whatsit introduces the children to the Happy Medium, a jolly woman in a silk turban and satin gown, bearing a crystal ball. Mrs. Whatsit asks the Happy Medium to show the children their home planet, but the Medium is reluctant to look at something so unpleasant. Meg, Charles, and Calvin see a vision of their planet in which it is surrounded by the Dark Thing that they first saw from the atmosphere above Uriel. Mrs. Which explains that the Dark Thing is the pure Evil that they will have to fight. She assures them that they are not alone; they join a legacy of warriors against the Dark Thing, the greatest of which have also been Earthlings: Jesus, da Vinci, Shakespeare, Einstein, Bach, and Gandhi. Impatient, Meg asks about her father; Mrs. Which informs her that he is held captive on a planet that has capitulated to the Dark Thing.
Whereas the last chapter was most clearly a statement of L'Engle's theology, this chapter presents her understanding of science. She wrote A Wrinkle in Time while studying Einstein's theory of relativity, which unites space and time in a single space-time continuum often likened to a fabric. This notion appears in the book when, in demonstrating how the group will travel through space-time, Mrs. Whatsit gathers together the fabric of her skirt.
This chapter also alludes to L'Engle's personal understanding of time, most clearly articulated in her autobiography A Circle of Quiet. She explains that there are two types of time: Chronos and Kairos. Whereas Chronos is ordinary clock time, divided evenly into hours, minutes, and seconds, Kairos is God's time, in which notions of past and present are irrelevant. When Meg fears that their mother will worry about her missing children, Mrs. Whatsit assures her that, due to certain properties of time, this will not be the case: the children do not travel through linear time on their journey to rescue Mr. Murry; rather, their quest is circular, involving an escape from ordinary Chronos into the realm of Kairos and then returning back to Chronos at a point prior to their departure. L'Engle's creative conception of time resembles the twin paradox and other notions that are a consequence of relativity theory.
Not only does L'Engle further develop the novel's ideas in this chapter; she also continues to present us with insights into Meg's character. Again we see Meg's desire to understand everything rationally: thus, when Mrs. Which starts to list the famous fighters of the Dark Thing, and the children add to the list from their knowledge of great cultural and historical figures, Calvin and Charles name religious leaders, painters, poets, and musicians, while Meg is able to list only mathematicians and scientists. Her invocation of Euclid and Copernicus reveals Meg's enduring commitment to conquering the world through rational thinking; she has not yet fully accepted the idea of explanations that exceed our logical understanding.
Meg is also reminded of the other lessons she has yet to learn: she must learn to be patient in spite of her desire to rescue her father immediately; she must learn moderation and compromise. Indeed, this latter challenge, first verbalized in Mrs. Murry's advice that Meg seek a "happy medium," and echoed by her twin brothers in Chapter 2, resounds in this chapter as a delightful play on words: Meg and her companions meet a jolly clairvoyant who is, in the most literal sense, a "happy medium." Though this is of course not what her mother had in mind, the medium is a playful realization of Mrs. Murry's words and perhaps proves Mrs. Murry wiser than she knows. The encounter also serves to connect Meg back to thoughts of her mother and her mother's advice; although Mrs. Murry cannot be with them, her words' abiding truth lingers with Meg comfortingly.