A Wind In The Door by Madeleine L'Engle is the direct sequel to A Wrinkle in Time. Now in school, Charles Wallace has to deal with bullying, in which the school principal, Mr. Jenkins, refuses to intervene. He also suffers from a strange illness that affects the "farandolae" living in his mitochondria, or so Mrs. Murry believes. Charles Wallace's claim of seeing dragons in the garden doesn't help Meg's worries either.However, it turns out that he is right—sort of. The "dragons" are in fact the cherubim Proginoskes, or "Progo" for short. Meg and Calvin also encounter Blajeny, a "Teacher" who informs them of the Echthroi, beings that seek to destroy the entire universe. The Echthroi are in the process of destroying Charles Wallace's farandolae. Meg, Calvin, Progo, and a disbelieving Mr. Jenkins are sent on a mission to combat the Echthroi and save Charles Wallace's life.
This book contains the following tropes:
Adults Are Useless: Subverted with Mr. Jenkins. He starts out as a perfect example of the trope but goes on to save the kids.
Bratty Half-Pint: Sporos, as little as a half-pint can get — he's a farandola, about the size of a single protein molecule.
Arbitrary Skepticism: Averted. When Progo and Blajeny show up, Meg, Calvin, and Charles don't question the presence of these aliens at all. Granted, they've been already been through some weird stuff already, but they trust Blajeny pretty fast for someone they just met.
Buffy Speak: Used and subverted. Calvin decides he's going to use fewmets (animal droppings, in this contect, those of dragons) as his pet cuss word from now on, causing a minor freak-out from Meg because Charles Wallace has been talking about meeting with dragons.
Does This Remind You of Anything?: The main conflict in the mitochondrion, between the nihilistic, individualistic, skeptical younger generation, who wants to overthrow the time-honored traditions, and who refuse to grow up, and the older generation, whose devotion to invisible truths and time-honored traditions is necessary for the continuation of their world. Given that this book was written in 1973, it sounds like this may be the author's view of The Sixties.
On the other hand, Meg recalls a conversation between her parents where they lament how humanity's insatiable desire for progress and technology has resulted in a world of pollution and violence, and the farandolae must be taught to listen to the music of the body they live in in order to reach harmony. Flower Power?
Evil Smells Bad: The Echthroi have a horrible stench, which only manifests when they aren't disguising themselves.
Heroic Sacrifice: Later Mr. Jenkins, who makes it out alive, and Progo, whose fate is ambiguous.
Honest Axe combined with Spot the Imposter: Meg is presented with three identical Mr. Jenkinses and asked to Name which one is the real one. She asks each one what he will do about Charles Wallace and his problems in school. The two fakers talk as though they want Charles Wallace to be blandly successful. She realizes the real Mr. Jenkins is the one who is annoyed by the test and doesn't get what's going on. Played straight in the end when the real Mr. Jenkins begins to understand Charles Wallace and Meg better, and he will work to make things better.
Our Angels Are Different: Proginoskes, the singular cherubim, who is a Deadpan Snarker composite of wind and flame at his heart, extending into dozens of immense wings and myriad, blinking eyes. (He finds it easier to not be corporeal at all, and scorns the human idea of "little pigs with wings.") Proginoskes' great skill is to Name people, and the key to naming is love.
Truth in Television: In point of fact, mitochondrial enzymes (protein molecules, or polypeptides) do go through a maturation process and many anchor themselves in the membrane. Having too many immature enzymes floating around too long is very bad for the cell and eventually causes it to self-destruct. L'Engle merely takes all the parties and applies the Anthropic Principle, turning them all into sentient beings, able to be influenced by the Echthroi.
Two-Teacher School: In the last book, Mr. Jenkins was Meg's high school principal; now he's Charles Wallace's elementary school principal. Justified by saying that the school board deliberately had him moved from one to the other because the elementary school needed reform.
That was just an excuse the school board used. In truth, he was being demoted because he couldn't handle the high school students.
Reptiles Are Abhorrent: Louise the Larger, the snake that lives in the Murry's garden wall, is not only harmless, she's described in quite handsome terms and, as a Teacher in her own right, is genuinely benevolent... none of which stops Charles Wallace's teacher from freaking out when he brings Louise to class for show and tell.