After his death, Jesus persuades his Father to give humanity another chance and send a new savior to them, so God leaves the only child not slain by Herod's troops at Mary and Joseph's doorstep to raise as their own. The boy's name is lost to history, and everyone simply knows him as the Redeemer, the Step-Son of God. Instead of Jesus' many miracles (walking on water, turning water into wine, making more fish, etc.), the Redeemer is given just one: the Word, a semi-magical power that allows him to put anything he desires into another dimension known as the Cold and retrieve it at a later time. He uses this power to prove his divinity and ascend to the throne of The Roman Empire.
Fast-forward to two thousand years later. The Roman Empire has never collapsed and now controls most of Europe and has colonies in Africa and the Americas, although it is now called simply "the State". The other powers in the world are the Russian Khanate (Russia never throwing off the Mongol yoke), the Chinese Empire (the most advanced nation in the world), the Ottoman Empire (by far the weakest), and the barely-mentioned Aztec Empire. The worldwide deficit of iron makes it the monetary standard, while gold is treated as a pretty trinket. Science and technology have stalled at roughly medieval levels, although certain aspects of The Renaissance and the Industrial Revolution are present. A number of people (mostly nobility) know the Word, although it is a much weaker version of the Original Word that the Redeemer knew, and use it to keep all their treasures safe from thieves.
The protagonist is a thief/tomb raider named Ilmar the Slick, who is captured and sent to an island Penal Colony to mine iron for the rest of his life. On the prison ship, he meets a young boy, who is revealed to be Prince Marcus, one of the illegitimate children of the Possessor, and soon finds out that the entire State is after the boy for unknown reasons. Ilmar helps Marcus escape, and they get to the mainland with the help (at gunpoint) of a glider pilot named Helen the Night Witch. Then they find out that anyone who has even seen Marcus has been either executed or imprisoned. Realizing they have no choice, they run, hoping either to escape or to turn in Marcus in hope that they will be spared.
The author pays special attention to the current state of the world, as well as how it got there from both historical and religious standpoints. The backstory is slowly revealed through recollections, sermons, and sometimes visions.
The duology contains examples of the following tropes:
- Alternate History: The world in the duology diverges from ours when Jesus is killed as baby to be replaced by another Messiah.
- The Antichrist: While the Church do not officially recognize the existence of a figure known as the Tempter, one of the gospels claims he will come before the Second Coming of the Redeemer claiming to bring good but secretly bringing evil.
- And he is either Marcus or Gerard.
- Bilingual Backfire: Ilmar and another character are taken on a tour of the town by a Magyar (Hungarian) teenager named Peter and try to talk amongst themselves in Gallic (French). Halfway through the conversation, Peter offers (in Gallic) to leave them alone, if they'd rather he not hear what they are saying.
- But Not Too Foreign: Main characters are not Russians, as opposed to many other Lukyanenko novels, but Russian spies appear later.
- A Child Shall Lead Them: A possibility with Marcus.
- The Church: This world has two Churches based in Rome instead of one. They are slightly different denominations but are both ruled by the Step-Son of God (the Pope). The Church of the Redeemer is more about punishing the wicked, while the Church of the Sister (one of the Redeemer's disciples) is about redemption for those who have strayed.
- Devil, but No God: Inverted. The official Church doctrine denies the existence of a counterpart figure whose power approaches that of God. To claim otherwise is heresy of the highest sort.
- Deliberate Values Dissonance: Despite having killed a number of people, Ilmar is never referred to as a murderer. While killing is a crime, the Church only recognizes killing over a dozen people (children and elderly are special cases) a mortal sin. In fact, it is allowed for one person to "take" someone's kill for himself (obviously, not in a legal sense).
- Ethical Slut: Helen, as well as most glider pilots, although she is the only female pilot. Her first crash resulted in a foursome with three other pilots, all of whom were just glad to be alive. When she crashes while transporting Ilmar and Marcus at gunpoint, she "recovers" by having sex with the immobilized Ilmar before leaving. She stops, though, after she starts sleeping with Ilmar.
- Grave Robbing: Ilmar's specialty is raiding ruins for possible treasure or artifacts. Being a devout follower of the Sister, though, he sometimes donates religious artifacts to the Church.
- Healing Hands: Bishop Gerard Lightbringer can cure cancer by what he believes is the power of prayer. In fact, what he is doing is removing the malignant cells and putting them into the Cold. However, this is no less miraculous, as putting anything of the living into the Cold is impossible for anyone other than the Redeemer.
- Hell: Unlike the Christian version of Hell as a fiery place of eternal torment, the version of Hell in the Redeemer's church is a frozen wasteland where time has no meaning. In fact, there is a strong possibility that Hell is the Cold.
- Hollywood Apocrypha: This reality's holy texts are read in a language similar to Old Church Slavonic, the Russian equivalent to the English in the King James Bible. Likely a Translation Convention, since everyone is supposed to be speaking Latin.
- Imperial China: Frequently mentioned but never shown. The Chinese Empire is the most advanced nation in the world, capable of building rocket boosters for their gliders that allow them to get all the way to the State in a single flight.
- Istanbul (Not Constantinople): Since Rome never fell, many European cities still retain their original Roman names: Budapest is Aquincum (despite the presence of the Buda, Óbuda, and Pest districts), London is Londinium, Paris (the capital of the State) is Lutetia. On a national level, Israel is still called Judea. Also, a number of European languages are called differently: Latin is called Romanian, Hungarian is called Magyar, Turkish is called Ottoman, Spanish is called Iberian, French is called Gallic, and Hebrew is called Judeic. This world's version of the Holy See is located in Urbis ("city" in Latin), which is, presumably, Vatican. Strangely, Vienna (Roman name - Vindobona) and Lyon (Roman name - Lugdunum) are still called as we know them.
- Like Brother and Sister: The official view of the Redeemer and the Sister's relationship.
- Love Interest: Ilmar and Helen appear to be growing closer throughout the duology, although each is driven by different motives. Subverted, Ilmar chooses not to follow Marcus at the end, while Helen becomes one of Marcus's disciples.
- Medieval Stasis: Much of the technology is unchanged from Medieval times, as the deficit of iron has prevented significant scientific progress. Electricity is a fairly recent discovery, although it has already been adapted to start glider engines (they previously used chemical-based starters). Guns (even automatic ones) are present but are very rare. Only officers and nobles are allowed to carry firearms. Wood-and-skin gliders, powered by weak engines and short-range rocket boosters, are the only means of air travel. Each glider can only carry two or three people. They are generally used to ferry messages, scout enemy lines or bomb fortifications. All pilots have to memorize wind charts in order to avoid crashing.
- Metal-Poor Planet: Earth itself was miraculously stripped of most of its minable iron in the backstory, leading to a civilization that is still struggling with industrialization around 2000 CE and where bronze swords are still common weapons.
- Modern Mayincatec Empire: The Aztec Empire is still around and occasionally clashes with the State's colonies in the Americas. The State's deficit of iron actually puts them on par, technologically, with the Bronze Age Aztecs.
- Moral Event Horizon: In-Universe example: The Redeemer's decision to become the Roman emperor led to all but one of his original twelve disciples (who are strongly suggested to have been the familiar Twelve Apostles, because the only one ironically remembered as loyal was Judas Iscariot) abandoning him. They were almost immediately replaced by eleven Roman legionnaires who witnessed the first ever use of the Redeemer's Word; interestingly, the twelfth witness (named Marcus) refused to join the rest, saying that he doesn't see God, only the next Emperor. Also, the Sister (probably Mary Magdalene), who is said to have been able to truly forgive anything, never forgave Redeemer for this, leading to the very first schism of The Church.
- Motifs: Just as the title suggests, the sky motif (not heaven, although in the original Russian the words for them are the same) is very prominent in the novels, probably symbolizing the inscrutable Powers That Be. It is probably no coincidence that every single time the characters take to the skies on a glider, they always come crashing down.
- Mundane Utility: The Word is a divine power granted by God to His Step-Son the Redeemer in order to spread the faith. What do the people choose to do with it? Keep all their stuff hidden from everyone else (i.e. the ultimate form of materialism). This is about as far as people can get here from God's plan, which is likely what Lukyanenko intended to show. At least once, this was used in battle. A Russian general named Kutuzov used his Word to put all his cannons into the Cold and then marched across mountains before putting them all into the field for a surprise attack on Napoleon.note
- Omniglot: Peter is fluent in eight languages, including his native Magyar (Hungarian), Roman (Latin, the language of the State), Gallic (French), Russian, Judaic (Hebrew), Ottoman (Turkish), Germanic, and Iberian (Spanish). He is also teaching himself Chinese.
- The Paladin: When Ilmar goes to a church to talk to a Church of the Sister official, he is sent to Urbis (Vatican) to talk to God's Stepson (the Pope). As an escort, an ordinary priest is given the temporary rank of paladin and a holy relic (a piece of the pole to which the Redeemer had himself tied to) to convince other Church officials that he really is on a holy mission. However, the carriage is intercepted by another paladin sent by the Church of the Redeemer with orders to kill Ilmar. The other paladin also has a holy relic, resulting in an impasse. Seeing no other way out, the paladins and their companions rush into battle. The paladins turn out to be equally matched and kill one another.
- "Pan Up to the Sky" Ending: Part two ends up with Ilmar looking into the sky above.
- Penal Colony: The Isles of Sorrow. Based on the described location, it's likely that the Isles are the Azores.
- Praetorian Guard: Several praetorian legions are mentioned in the books. They are elite troops, consisting entirely of nobility and armed with the best weaponry, including guns. At the end of the book, two such legions are about to come to blows: the Grey Vests, the best of the praetorian legions of the State, and the Semetskiy guard regiment, an elite unit from the Russian Khanate.
- Practical Currency: Iron is used as the currency standard due to its rarity but has plenty of obvious uses.
- Ragtag Bunch of Misfits: We have a religious tomb raider, a flying ace, a police captain, a retired obstetrician, a noblewoman-turned-nun, a runaway monk, a retired pilot-slash-reluctant writer, a robber-turned-bishop, a polyglot, a spymaster, and a poor fisherman all following a 14 year old bastard prince. What a crew. Notably, all but one are followers of the same faith. The fisherman is Jewish.
- Redemption Equals Fate Worse than Death: The Redeemer chose to confine himself to Hell because of the realization that the moment he stepped onto the Roman throne, he failed the mission he was given by God. Thus, the Step-Son of God couldn't become the Son of God, who refused the temptation of worldly power and died horribly, only to truly redeem humanity. Ironically, the popular view of him was exactly the opposite.
- By the end of the duology, Marcus seems to have committed the same mistake (assuming he is really the Second Coming of the Redeemer rather than the Tempter) and Ilmar is the only one who realizes it. Marcus appears to realize it, too, after Ilmar's "betrayal", but he is already too far gone to turn back.
- Religion is Magic: The Word allows anyone who knows it to put any personal object into another dimension known as the Cold and retrieve it at any point. It is seen as a divine power, not magical.
- Richard Nixon, the Used Car Salesman: Several of the characters encountered by Ilmar appear to be alternate versions of known people in our world.
- Arnold, a huge Austrian man, is a trigger-happy officer of the Guard, determined to hunt down a teenage boy.
- Count Antoine of Lyon is a retired glider pilot who writes poetry in his spare time and has plenty of old stories he can share, including one where his glider crashes in the desert.
- Bishop Gerard Lightbringer, according to Word of God, is supposed to be a duplicate of Gérard Depardieu, although nothing in the books appears to point to that.
- Schizo Tech: Bronze swords, spears, and bows coexist with revolvers and machineguns. Horse-drawn carriages and sailing ships coexist with wooden bomber gliders and rocket boosters. It's all explained by the deficit of iron.
- Somebody Set Up Us the Bomb: In this reality, the Gunpowder Plot was successful, resulting in the British Isles becoming a part of the State.