In chapter five of Madeleine L'Engle's A Wrinkle in Time, the three children (Charles Wallace Murry, Meg Murry, and Calvin O'Keefe) are being taken to see something by their new friends, Mrs. Whatsit, Mrs. Which, and Mrs. Who. The three rather odd ladies (supernatural creatures in disguise)+ are able to transport the children through something they call a tesseract. The shortest distance between two points is not a straight line but a tesseract. Imagine holding a piece of string, each end in one of your hands. Instead of trying to traverse the line, you can close the gap between your fingers and the distance has been dramatically and instantaneously shortened.
The women show the children something in the distance, a kind of dark and smoky haze which is drifting across the atmosphere. It crosses a planet as they watch:
The outlines of this planet were not clean and clear. It seemed to be covered with a smoky haze.
It is the Dark Thing, and Meg asks:
"That dark Thing we saw.... Is that what my father is fighting?"
"YES," Mrs. Which said. "Hhee iss beehindd thee ddarrkness, sso thatt eevenn wee cannott seee hhimm."
Mrs. Whatsist adds:
"Your father needs help, he needs courage, and for his children he may be able to do what he cannot do for himself."
The reality is that the Dark Thing is a form of evil which has been around for centuries, and many great figures in history spent their lives trying to battle it. People like Jesus, Euclid, Shakespeare, Madame Curie, Michelangelo, and Buddha have fought back the darkness, and now Meg's father has joined the fight. Doctor Murry has been away from home fighting the Dark Thing, and now he needs help. He is struggling and needs help; his children will have to help save him and earth from the invading darkness.