On a dark and stormy night, teenager Meg Murry is lying in her bed. It's not only the stormy weather that's keeping her awake—she can't stop thinking about how much of a misfit she is at school, and she can barely think about her father (who's been missing for a year) without crying. There have also been tales of a tramp on the loose who stole the constable's wife's sheets—and this is just the kind of night for a tramp to be wandering around The weather and her gloominess send her down to the kitchen in search of hot cocoa, where she finds her five-year-old brother, Charles Wallace, waiting for her.
While her missing father provides ample reason for Meg to be upset, she seems almost more unhappy about being a misfit at school because she doesn't accept that her differences are good and loveable and ought to be embraced.
Charles Wallace is another cause of Meg's concern. He's unusually intelligent (not surprising, as both their parents are brilliant), but he also has a strange sense of intuition with which he can tell how Meg and her mother are feeling. He's bullied because he's different and Meg, who loves him fiercely, will try to beat up anyone who makes snide comments about him. Just this afternoon she got into a fight with an older boy who did exactly that. She wishes she were more like her younger twin brothers, Sandy and Dennys, who are nice and normal and are never criticized by anyone at school.
Although Meg loves Charles and values his differences, she still wishes that they could both be normal like their twin brothers, Sandy and Dennys. She doesn't yet realize how good the differences are that she and Charles have been endowed with, or the importance of difference in general.
Anticipating her thoughts, Charles has already put some hot milk on the stove for Meg. As they chat and make sandwiches for themselves, Mrs. Murry joins them. She's almost extravagantly beautiful—unlike Meg, who is plain with mousy brown hair—and loves her children tenderly. As she takes a look at a bruise that Meg got earlier that day from getting in a fight, she gently tells her daughter, "A happy medium is something I wonder if you'll ever learn."
Meg also hasn't yet come to appreciate what a wonderful thing it is to have a mother who loves her the way she is and who tries to guide her towards a proper acceptance of herself.
Suddenly, their dog, Fortinbras, growls at the door (it's still storming furiously outside), and Mrs. Murry goes out to investigate, bringing back in an old woman covered in an absurd amount of brightly colored clothing. Charles seems to know her and calls her Mrs. Whatsit. Meg is very suspicious of her, considering her strange clothes and the time of night, but makes her a sandwich as well nonetheless.
Fortinbras is actually a character from Shakespeare's Hamlet, the first of many classical allusions in this book. Also, judging Mrs. Whatsit from her weird looks, she's a crazy old woman, but she will prove to be something altogether more interesting and powerful.
As Mrs. Murry calmly helps Mrs. Whatsit to take her water-filled boots off, Mrs. Whatsit reveals out of the blue that she was the "tramp" that stole the constable's wife's sheets, and furthermore, that there is such a thing as a "tesseract". While Charles and Meg have no idea what this means, Mrs. Murry turns white and murmurs to herself, "How could she have known?" With that, Mrs. Whatsit disappears back out into the night.