The Koryfonic Empire sits in the center of the multiverse. That is because the multiverse itself is shaped as an infinity sign, with the Koryfonic headquarters on the center of it all, its empire ruling across several worlds with an iron fist. Its domains include both sides of the multiverse. One side is "Ayewards", in which magic is common practice and easy to do, while the other is "Naywards", the worlds in which magic is nothing more than superstition, if not completely unknown. Our Earth is currently on the Naywards side, and seems to have been sliding back for a while.
Keeping watch over the balance of magic, a group of persons of great power called the Magids have a great number of agents on each one of the worlds, Ayewards and Naywards alike. One of the Magids from Earth, Stan, passes away. His successor Rupert Venables needs to find a successor himself to replace his late mentor. Among them he finds a great prospect on the person of Maree Mallory; however, he is horrified to find that she doesn't seem to be ethical enough for the Magid job, and all other candidates are on different parts of the globe. Looking for a way to contact them all, he meddles with the fates of each one to get them all together at a Fantasy Convention at Hotel Babylon near where he lives, but they are not the only ones to show up.
Not being in enough trouble, Rupert is also suddenly called to the Koryfonic Empire. A terrorist attack blows up the palace and kills the Emperor and most higher-ups, and odd customs make finding the heirs very hard, what with most of them already being dead and the others protected by a very secure system. Rupert gets his hands full with both jobs as he searches for the missing heirs and searches for a Magid successor, and things start getting out of hand quickly.
The book has a lot of tense situations while still poking fun at its own nature and, of course, at fantasy conventions themselves. It also is, unlike many of Jones's novels, written for adults and has no qualms on mentioning sex and featuring horror.
Deep Secret was followed by The Merlin Conspiracy, featuring the further adventures of Maree's cousin Nick. The second book is somewhat tamer than the first and can be enjoyed without having read the first, though some backstory may be missed. It is also less definitely aimed at young adults; this is part of the reason it was not advertised as a sequel.
Tropes appearing in Deep Secret include:
- Alien Geometries: The Hotel Babylon has halls where you can go around more than four right angles before coming back where you started, thanks to the building being on top of a bunch of ley lines. And someone is toying with the node that the hotel is set on.
- Alliterative Family: Rupert's brother Will Venables has six daughters, all of whose names begin with the letter V.
- All Part of the Show: After the action moves to Phantasmacon, a lot of the magical goings-on go unremarked because the con-goers assume they're just part of the convention, with a high point being an injured centaur passing through the busiest part of the convention with the excuse that it's preparation for the costume competition.
- Amicable Exes: Rupert and Zinka used to date. They broke up many years ago, but are still very good friends and still stick out for each other.
- Amnesiac God: It's eventually revealed that Rupert's rather boring, bland neighbor Andrew is actually one part of Emperor Koryfos, who was split into multiple people thousands of years ago. Once returned to his full power, Koryfos then reveals himself to be one of the Powers That Be.
- Aura Vision: One of Rupert's Magid candidates is a new-age-ish woman who claims to be able to read auras and keeps button-holing people and telling them their aura is in a terrible state. After spending some time with her, Rupert determines that she really does have Aura Vision, but her own psyche is so messed up that she can't see through it when she tries to read other people.
- Beautiful All Along: A variant of sorts. Maree doesn't actually get any more physically attractive, apart from a haircut and change of clothes, and she was never a looker to begin with. However, due to both understanding her personality better and the fact that she's very nearly just come Back from the Dead, Rupert perceives her as more beautiful, and that's good enough for him.
- Belligerent Sexual Tension: Rupert and Maree. Both of them having Jerk with a Heart of Gold tendencies helps nothing.
- Big Beautiful Woman: Zinka. Maree describes her as "plumpish" (but then, Maree's not small herself), and Rupert refers at one point to her "lovely, ample form".
- Blessed with Suck: If people with enough magic power to become magids aren't taught to harness it properly, it has bad effects on their mind, either making them insane or just jerks.
- Bumbling Dad: Nick's. Despite being a famous horror writer, he absolutely does not understand his teenaged son.
- Carnivore Confusion: Everyone wonders if the centaur Rob eats meat. (He does.)
- Chubby Chaser: Rupert seems to go for larger women.
- Cloud Cuckoolander: Andrew, though there's a reason for that.
- The Conqueror: The Koryfonic Empire was founded by Koryfos, a pseudo-Alexander the Great who conquered entire universes. By the time of the story, his empire has dwindled and is on the verge of total collapse.
- Face Doodling: Pulled by Zinka on a poor slob left drunkenly passed out on the stairs. Rupert notes that the vines she's drawing will bring him under her power.
- Family Relationship Switcheroo: One of these occurs when the identities of the Emperor's heirs are uncovered. Nick and Maree, who thought of themselves as cousins, are actually half-siblings, as they're both children of the Emperor by different mothers. Janine is Nick's real mother, but Ted is only his stepfather.
- Fan Convention: The main setting for the latter part of the novel is an sf convention called Phantasmacon.
- Fantastic Racism: Thanks to being brought up on an isolated compound, Rob is confused about why he should care about the deaths of people outside his family. Will sets him straight.
- Geas: Rupert defeats one antagonist by putting him under a geas not to cast magic. If broken, he would die. He does.
- Good Bad Girl: Zinka rather gets around (and makes spending money while on Earth by selling her, uh, interesting artwork at conventions), but she's quite well-respected and generally considered a really good person by everyone who meets her.
- Guardian of the Multiverse: The Magids take instruction from a group of Powers That Be known as "the Upper Room".
- Half-Human Hybrids: Rob the centaur, and he's not the only centaur of the sort.
- Happily Adopted: Nick and Maree. Neither of their "fathers" is actually related to them. Maree starts calling her stepfather her "As-it-were Dad" and still loves him immensely. Nick has some difficulty getting to know his stepfather for real, but in the end, he's still better than Janine.
- Hidden Backup Prince: All of Emperor Timos's children are raised in hiding, partly to prevent his enemies taking advantage of them and partly to prevent them getting any ideas about speeding up the succession. This causes problems when an explosion kills the Emperor and destroys the records of where his children are hidden, leaving Rupert with the job of figuring out who and where his heir is.
- Hot Consort: High Lady Alexandra, one of the only survivors of the assassination of Emperor Timos and his many wives and consorts, was one of the least important of his consorts. Given how beautiful and intelligent she is, Rupert speculates that after all his political marriages were accomplished, the emperor started picking ones like her for his own amusement.
- Hot Guy, Ugly Wife: Rupert is repulsed by Maree at first, her being chubby, a bit smudgy, and her voice being rather whiny. Maree's old boyfriend was rather handsome, and she of course ends up with Rupert, who is fairly good-looking and does love leather jackets and other stylish clothes. By the time she comes back from Babylon, he's convinced there's no more beautiful woman in all the Multiverse and wouldn't dream of having anyone else. She's just that plucky, no one can resist her.
- I Am Not Your Father: Nick is rather shocked to learn that his mom is the Big Bad and the man who raised him is not his real father—in fact, his birth dad is the recently-murdered Koryfonic Emperor. He's visibly relieved when he finds out that obviously his adoptive father still plans to raise him, despite his mom's death.
- Immune to Fate: Magids, to an extent. They're removed from the "fatelines" or so Rupert Venables thinks once they're made Magids, but Them Up There can still mess with them.
- Interspecies Romance: Rob is half centaur. Centaur/human interbreeding is described as being a very bad idea, though, with the babies often stillborn (always, if the human is the mother). It's also indicated that the joining of Rob's parents wasn't romantic as such, but part of a political alliance.
- Jerk with a Heart of Gold: Nick, Rupert, and Maree all have rather bitter exteriors hiding kind hearts. In all three of their cases, it's mostly a defense mechanism—Rupert against his job, and Nick and Maree against their mother/aunt. Nick also has the excuse of being fourteen.
- King in the Mountain: There's a prophecy that the first Koryfonic Emperor — Koryfos himself — will return when the Empire is in grave need. He shows up at the end of the novel, at the head of a detachment of Imperial troops, just in time to help defeat the villains.
- Ley Line: Rupert is very distressed when he realizes the hotel where he's gathering his magical candidates is built at a node between ley lines. It has a bit too much magical power behind it for him to be comfortable working with it.
- Love at First Sight: A variation. Rupert realizes, after meeting the real Maree for the first time, that he was deeply infatuated with her just from the vibe she gave off in the letter she wrote back to him. Repulsed by her appearance, he changes his mind but after spending more time with her, he realizes that he loves her anyway.
- Magic Dance: The Witchy Dance is an old kids' game that Maree and Nick used to do to calm down, turns out it has actual power. It's been keeping the local hedge goddess from completely subsuming Maree and is important in the climax in raising the power needed to danish the goddess for good and reinstate the emperor Koryfos.
- Meet Cute: And lord, what a meeting it is...
- More than Mind Control: Nick's mother uses this on him. (He's mostly immune by the time the story takes place).
- Must Have Caffeine: Nick is a classic example of this trope.
- No Blood Ties: In centaur culture, like in Real Life matrilineal societies, a male centaur is expected to be more loyal to his sister's children than his own. This is a plot point, as it explains Rob's upbringing: the (human) emperor was his father, and his uncle brought him up.
- No Celebrities Were Harmed:
- Neil Gaiman (or, at least, an unnamed famous author in a leather jacket whom Maree thinks is good-looking) makes an appearance at the convention.
- In his memorial post about Diana Wynne Jones, Neil also says that Nick's Must Have Caffeine (and the sleep eating) were based on him.
- Not a Morning Person: Nick is very much not a morning person. Not only is he grumpy, he can't even speak coherently or open his eyes until he's had four cups of coffee.
- Odd Name, Normal Nickname: Nick Mallory's full first name is revealed to be Nicothodes.
- Our Archons Are Different: They are part of the make up the 72 beings that run The Upper Room, the local Powers That Be while also being a Council of Angels. Some take on human form and others do not. The emperor Koryfos is one of the form.
- Our Centaurs Are Different: Centaurs turn out to be very central to the plot, after they first appear roughly halfway through. They're standoffish and proud with a distinctive set of cultural norms, and they, like other magical creatures, can no longer live in this world for long since it's less magical than it used to be. Their human halves are the same color as their horse parts, and their culture is clannish and matrilineal. Female centaurs can interbreed with humans, though it's noted that a human woman could not carry a male centaur's child.
- Out-of-Clothes Experience: On the journey to Babylon, Nick feels a thorn cut his stomach and suddenly realizes that all of their clothes have disappeared (which sucks extra, given that it's freezing and the ground is rather rough). A short while later, they find trees filled with any clothes that they've ever given away in the past, and dress in those.
- Perception Filter: Magic-users can put "don't-notice" workings on objects, though it works only as a weak Weirdness Censor.
- Place of Power: Hotel Babylon is built right on top of a power node, giving it hallways that turn ninety degree angles and keep going on past the point of a perfect square, a clear path to the next dimension over, and a hotel full of people unconsciously attracted to the fan convention it's hosting. (Admittedly, Rupert's fiddling around with fate may have something to do with the last one.)
- Plucky Girl: Maree, to the point where even losing half her soul won't stop her.
- Powers That Be: The Upper Room, the group that guides the Magids, is a mixture of senior Magids who have been promoted into the group and of greater-than-human Powers. The emperor Koryfos is one of the latter.
- Production Foreshadowing: The characters mention a demon designed by horror writer Ted Mallory, who is blue, has three legs, smells of ozone and feels acidic. A demon fitting this description is a character in Dark Lord of Derkholm which was published a year later.
- Prophecies Are Always Right: The prophecy about Emperor Koryfos returning when the Empire was in great need comes true near the end. Rupert's superiors mention that not all prophecies are right, but this one happened to be from a reliable source.
- Quest for a Wish: Maree and Nick's journey to Babylon to find a wish-granting entity that can heal Maree after half her soul was murdered.
- Rags to Royalty: Nick, Rob, and Maree are all royalty. Nick and Rob know; Maree doesn't, and when she finds out, she is NOT pleased.
- Rescued from the Underworld: The quest to Babylon to restore Maree's soul. Babylon isn't the land of the dead exactly, but it's definitely "outside here or there", and it's associated with death in the folk songs and myths which contain echoes of it.
- Rule 34: Actually invoked in-story, as the character of Zinka likes to sell her... interesting paintings at the convention.
- Save Both Worlds: If the villains are successful, it will be bad not only for their own world, but also for many others including Earth.
- Scary Shiny Glasses: Rupert's glasses are described as doing this at one point when he's especially angry at someone.
- Screw Destiny: Rupert finds himself thinking this a lot.
- Selfless Wish: When Maree and Nick reach Babylon, Maree is supposed to wish for the other half of her soul back, but instead wishes for her dad to be cured of cancer, so Nick, in turn, has to use his wish on her.
- Small Name, Big Ego: Mervin Thurless, one of the guests at the convention.
- Spirit Advisor: Stan gets a dispensation to stick around as a ghost and help Rupert find his successor.
- Succession Crisis: The Koryfonic Emperor is so paranoid that his children will overthrow him that he has them all hidden away with adoptive families; at one point he even executes a son who accidentally found out the truth. Then a bomb goes off in his palace, killing him and most of his wives/consorts, and as the worlds-spanning empire descends into chaos the protagonist has to track down someone capable of taking the throne.
- Twirl of Love: Rupert and Maree, when they're reunited near the end.
- Would Hurt a Child: Janine would murder several.
- You Are in Command Now: Dakros, a mid-ranking military official, winds up in charge of the entire Koryfonic empire when the Emperor, and almost all of his high-ranking staff, are assassinated. Rupert even suggests that he take over as Emperor for good, but Dakros vehemently refuses.
