A Wind in the Door is a 1973 novel by Madeleine L'Engle.

The story follows the adventures of the Murry children and Calvin O'Keefe, whom we first met in A Wrinkle in Time. Cosmic evil is connected with evil on a cellular level, and the children along with some new friends go within the mitochondria of Charles Wallace in order to save him from the un-namers -- the echthroi. A memorable new character, a cherubim named Progonoskis, who seems to resemble a Seraph, becomes a particular friend of Meg's, and she learns to appreciate and embrace inner beauty and strength. Like all of L'Engle's books, the power of love is again a force to be reckoned with, and science and spirituality are intertwined. Trees speak to one another on the wind.