''A Wind In The Door'' by Creator/MadeleineLEngle is the direct sequel to ''Literature/AWrinkleInTime''. 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 [[InsistentTerminology 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:
* AdultsAreUseless: Subverted with Mr. Jenkins. He starts out as a perfect example of the trope but goes on to save the kids.
* AmbitionIsEvil: When Meg is mystified at the actions of the farandolae, Mr. Jenkins explains that, for some, ambition is an end in and of itself.
* BetterToDieThanBeKilled: Proginoskes makes it clear that there is a great difference between being Xing oneself and being Xed by the Echthroi. To be Xed by the Echthroi is (it seems) to be turned into an Echthros, but to X oneself is to give oneself "all the way away" to creation, and no one really knows what happens to someone who Xes themselves, or whether or not it lasts forever.
* BigBad: The Echthroi. Their goal is to make '''everything''' into '''nothing'''.
* BrattyHalfPint: Sporos, as little as a half-pint can get -- he's a farandola, about the size of a single protein molecule.
* ArbitrarySkepticism: 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.
* BilingualBonus: The mitochondrion that the final conflict takes place in is called Yadah. In Hebrew, it means "worshipping with hands outstretched" or in more modern usage, "Lifting up one's voices to sing God's praises." The name was no accident.
* BuffySpeak: 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.
* CallBack: During the SpotTheImposter quest, [[spoiler: a fake]] Mr. Jenkins alludes to a conversation that took place in AWrinkleInTime.
* DoesThisRemindYouOfAnything: 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 TheSixties.
** 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. This sounds more like the counterculture's desire for environmentalism, simple living, and mysticism.
* EvilSmellsBad: The Echthroi have a horrible stench, which only manifests when they aren't disguising themselves.
* FantasticVoyagePlot: A very... metaphysical example, but the characters do ultimately fix the situation by entering Charles Wallace's body.
* FindTheCure: Meg's overarching goal is to cure Charles Wallace, except that there is no external cure -- the only solution is to go into his cells and attack the disease directly.
* GentleGiant: Blajeny.
* HeroicSacrifice: Later [[spoiler: Mr. Jenkins, who makes it out alive]], and [[spoiler: Progo, whose fate is ambiguous]].
* HonestAxe combined with SpotTheImposter: 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 [[spoiler: the one who is annoyed by the test and doesn't get what's going on.]] Played straight in the end when [[spoiler: the real Mr. Jenkins begins to understand Charles Wallace and Meg better, and he will work to make things better.]]
* JerkWithAHeartOfGold: Mr. Jenkins. The book deals with Meg having to overcome her grudge and see the goodness in him.
* ObstructiveBureaucrat: Mr. Jenkins, at first, to compensate for his weak-willed nature.
* OurAngelsAreDifferent: Proginoskes, the singular cherubim, who is a DeadpanSnarker 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 [[ThePowerOfLove love.]]
* OurGiantsAreBigger: Blajeny is very big, from another planet, and a Teacher (as ordained by the universe). He's also a GentleGiant.
* {{Synchronization}}: Kything has elements of this, as seen with the scientist's experiment with his plant.
* ThePowerOfLove: Just as prominent as the last book.
* PowerOfTheVoid: The Echthroi, whose powers manifest as holes in reality.
* TruthInTelevision: 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 AnthropicPrinciple, turning them all into sentient beings, able to be influenced by the Echthroi.
* TwoTeacherSchool: 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. [[spoiler:That was just an excuse the school board used. In truth, he was being demoted because he was too weak-willed and couldn't handle the high school students.]]
* ReptilesAreAbhorrent: Played with. 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.
* WarIsHell: Mentioned briefly, though not quite as prevalent as [[Literature/ASwiftlyTiltingPlanet it would later become]].
* WithAFriendAndAStranger: A variation from the previous adventure; this time Calvin O'Keefe is the friend, and Meg's partner Proginoskes is the stranger. Charles Wallace is excused from this CastCalculus on account of his illness.
* XMakesAnythingCool: Unnaming, the ability of the Echthroi, is also referred to as Xing. Xing is categorically a terrible thing, but it ''does'' sound pretty cool.
* YearInsideHourOutside: Within a mitochondrion, the heart of the host ([[spoiler: Charles Wallace]] in this case) beats about once a decade.