One night, thirteen-year-old Meg Murry meets an eccentric new neighbor, Mrs. Whatsit, who refers to something called a tesseract. Meg later finds out it is a scientific concept her father was working on before his mysterious disappearance. The following day, Meg, her child genius brother Charles Wallace, and fellow schoolmate Calvin visit Mrs. Whatsit's home, where the equally strange Mrs. Who and the voice of the unseen Mrs. Which promise to help Meg find and rescue her father.
Mrs. Whatsit, Mrs. Who, and Mrs. Which turn out to be supernatural beings who teleport Meg, Charles Wallace, and Calvin O'Keefe through the universe by means of a tesseract, a fifth-dimensional phenomenon explained as folding the fabric of space and time; this form of travel is called tessering. Their first stop is the planet Uriel, a world inhabited by centaur-like beings who live in a state of light and love, fighting against the approaching darkness. There, the Mrs. Ws demonstrate to the children how the universe is under attack from an evil being that appears particularly clearly on Uriel as an overwhelming dark cloud, called The Black Thing. They then take the children to Orion's Belt to visit the Happy Medium, a far-seeing person with a crystal ball through which they are shown that Earth is partially covered by the darkness, although great religious figures, philosophers, scientists, and artists, have been fighting against it. Mrs. Whatsit is revealed to be a former star, who exploded in an act of self-sacrifice to fight the darkness.
The three Mrs. Ws tesser the children to the edge of the inhabited part of a dark planet named Camazotz, which has succumbed to The Black Thing, and where the Mrs. Ws cannot themselves enter. Meg's and Charles Wallace's father, Alex Murry, is imprisoned in a nearby city because he refused to yield to the group mind that causes inhabitants to behave in a mechanical way. When they reach the CENTRAL Central Intelligence building, Charles Wallace deliberately allows himself to be hypnotized, in order to find where their father is kept. Controlled by the group mind, Charles Wallace takes Meg and Calvin to the place where Meg's father is being held prisoner. He then takes them to IT, the disembodied brain with powerful telepathic abilities that controls the planet. Using special powers from Mrs. Who's glasses, Meg is able to reach her father, who tessers Calvin, Meg, and himself to the adjacent planet Ixchel, before IT can control them all. Charles Wallace is left behind, still under the influence of IT, Meg is paralyzed from contact with The Black Thing during the trip, and Dr. Murry suffers broken ribs. The inhabitants of Ixchel are beast-like, with featureless faces, tentacles, and four arms. Despite their frightening appearance, they prove to be both wise and gentle; one cures Meg's paralysis, prompting Meg to nickname it "Aunt Beast".
The trio of Mrs. Whatsit, Mrs. Who, and Mrs. Which arrive on Ixchel, before Alex Murry has recovered, and assign the rescue of Charles Wallace to Meg alone. Arriving at the building where IT resides, she finds Charles Wallace still under IT's control. Inspired by hints from the Mrs. Ws, Meg focuses all her love on Charles Wallace and is able to free him from IT, at which point Mrs. Which remotely tessers Meg and Charles Wallace off Camazotz.
They all then tesser back to Earth, to the edge of the forest near the Murry home, and back to the moment in time just after the Mrs. Ws and the children originally left Earth. Then the Mrs. Ws vanish.[b]
