A Wind In The Door is a  fantastic adventure story involving Meg Murry, her small  brother Charles Wallace, and Calvin O'Keefe, the  chief characters of A Wrinkle In  Time. The seed from which the story grows is the  rather ordinary situation of Charles Wallace's  having difficulty in adapting to school. He is  extremely bright, so much so that he gets punched around  a lot for being "different." He is also  strangely, seriously ill (mitochondritis -- the  destruction of farandolae, minute creature of the  mitochondria in the blood). Determined to help  Charles Wallace in school, Meg pays a visit to his  principal, Mr. Jenkins, a dry, cold man with whom Meg  herself has had unfortunate run-ins. The interview  with Mr. Jenkins goes badly and Meg worriedly  returns home to find Charles Wallace waiting for her.  "There are," he announces, "dragons in  the twins' vegetable garden. Or there were. They've  moved to the north pasture now."

  Dragons ? Not really, but an entity, a being  stranger by far than dragons; and the encounter with this  alien creature is only the first step that leads  Meg, Calvin, and Mr. Jenkins out into galactic  space, and then into the unimaginably small world of a  mitochondrion. And, at last, safely, triumphantly,  home.