There is a world in which Rome never fell, and nobody has ever imagined that it could possibly have been otherwise. The Scriptor family governs the continent of Northern America from their villa on an island known in another history as Manhattan. Marcus Americanus Scriptor is born on the eighth day of Januarius in the year MMDCLXI, his birth induced to occur on what the family’s astrologers have determined will be the most important date in Roman history. On the same date, an old man appears out of thin air in the middle of Rome’s Forum, and is set upon by a mob of frightened citizens who witnessed his sudden materialisation. The old man is rescued by two passing centurions, but in the confusion he loses a remarkable item of jewellery which is later confiscated by the local magistrate. Unable to communicate with the old man, who does not seem to recognise maps of Rome or of Europa, the magistrate washes his hands of the affair and has the old man sent to a sanatorium in the Alps.

Marcus is an inquisitive child and historical prodigy, and at 12, he writes a three-volume history of the native Americans. At his father’s request, Marcus then joins the army and is sent to Britannia for training, where he is later joined by his younger brother, Titus. When Marcus is 18, he receives a letter from the Emperor, who has read his history and is mildly impressed. Marcus dashes off a quick letter pointing out that he was still a child when he wrote it, and despite the presumption of his letter, the Emperor commissions him to write a history of the Hercules Bridge that spans the strait between Hispanus and Africa. The work is a smashing success, which Marcus follows up with a history of the Roman Forum. In the course of writing this history, he makes many important contacts, including a Scotic aristocrat, Tertius Angus Inducula, whose daughter Angela is of marrying age. The families arrange for Marcus and Angela to meet, and their betrothal and marriage soon follows.

Through Marcus’ history of the Forum, he learns about the mysterious old man, and when he investigates the odd story he tracks down the missing bangle, which the late magistrate had bequeathed to his mistress. Marcus arranges for the old man to be brought to Rome, where he finds that they can now communicate clearly, as if the bangle is translating for them. The old man seems to be looking for somewhere to escape to, which Marcus finds odd, as he can’t conceive of anyone wanting to escape from the utopia that is Rome. The old man then tries to compel Marcus to hand over the bangle, and as they struggle, they are somehow transported to a building where the centurions are equipped with metal boxes that speak to them. The landscape outside is clearly that of Rome, but the architecture of the city is completely different.

The old man tries to take advantage of Marcus’ confusion to escape, but Marcus pursues him and cuts off his arm. The bewildered centurions now confront Marcus, but he easily defeats them, killing one and injuring the other. He seizes one of the centurions’ talking metal boxes, and also takes the bangle from the dying old man -- and then unexpectedly finds himself back in his own Rome. Marcus discusses the experience with his family and friends, and concludes that he has trespassed and committed murder on Roman soil, which will invite repercussions. However, since the old man had been here for 20 years and nobody tried to rescue him in that time, logic suggests that not everyone from the other Rome has the ability to travel between worlds. Either that, or the old man was from yet another world -- in which case, just how many are there?

Marcus, his father-in-law, and the scribe Laton use the bangle to visit the other Rome and scout out this strange new territory. Much is familiar, but many other things are not, including the “shiners” that transmit sounds and music, and the flying vehicles called helicopters. Before returning to “his” Rome, Marcus buys a book about the other Rome’s history, and learns that its history diverged from his own when a Tuscan genius named Leonardo invented flying machines and discovered a method of transmitting sound over great distances with a “shining wave.” The Romans used the new technology to spread rapidly across the world, but in so doing they encountered many new diseases and spread them across the world as well. The other Rome is now an isolated, walled community, its inhabitants terrified of outsiders bearing disease.

Marcus decides to make a diplomatic approach to the other Rome, which requires the approval of Emperor Emmanuel Victorius. Marcus claims that the bangle was found in America, and when he demonstrates its use, the Emperor agrees to contact the other Rome. Marcus thus negotiates a trade deal with the other Rome’s Emperor, Julius Mundus. Emperor Victorius insists that he and the other Emperor must be treated as equals, but for the sake of convenience, Marcus and his fellow travellers always refer to Marcus’ home as Roma I. It soon becomes clear that the bangle can travel in four “directions,” and that there are many other versions of Rome out there, including Romes where women are in charge of politics and Romes influenced more by the Aegyptian culture than by the Greeks. The Scriptor family grows in power and influence back in Roma I as Marcus and his allies forge further contacts and trading agreements with the other Romes.

When the explorers contact Roma CLII, they find it a strange world in which giant lizards the size of dragons walked the Earth and were tamed by the Romans -- but the world suffered a terrible catastrophe shortly before Marcus and his allies arrived. The sky has been shrouded in smoke for months, all of the plants and “thunder lizards” have died, and the human population of the planet is now numbered in the hundreds. One of the few survivors, a woman named Regina, theorises that a similar catastrophe may have occurred on the other worlds, wiping out their thunder lizards before human history began. While exploring Roma CLII, Marcus encounters a terrible shape-shifting creature with a single eye that attacks him and tries to seize his bangle; apparently the old man and his twelve friends were part of a larger conflict and may have come here in order to make an illegal move against their enemy, but the creature has tracked them down to kill them and take their bangles. But while the creature is prepared for any ritual or time-active weapon that Marcus can throw at it, it isn’t expecting him to attack it with something as primitive as a sword. Marcus pierces its eye, and the creature explodes in a burst of radiation, leaving behind only the seven bangles that it had already stolen.

The Scriptors keep one of the eight bangles in storage for safekeeping and send another to be studied in the hope of discovering its secrets; the other six are used to travel between the worlds. Freed from the necessity of transporting others between worlds, Marcus now has the chance to spend time at home with his wife. While doing so he learns that the Scriptors are accumulating enemies at court who are jealous of their growing power. By now, Marcus has seen enough different versions of Roman history to understand that Emperors are by now means above petty politics, and he thus arranges to present the Emperor with a gift: an erotic statue of Venus with a microphone hidden inside. The Scriptors are thus able to eavesdrop on his private audiences and learn who their enemies are, and they use the bangles to move against them, bypassing their security by moving through space in a neighbouring version of Rome. Some they blackmail, some they kill, until their power is secure. The Emperor soon realises what they’ve done, and, angered, he publicises the truth about the time bangles and the other versions of Rome. The news is too big for the public to handle at first, and since the Scriptors still have exclusive access to the bangles in any case, life goes on much as before. However, as the implications of the Emperor’s announcement begin to sink in and people realise that their world is not unique, they begin to question for the first time whether Rome’s triumph truly is predestined by the gods, or whether they really are living in the best of all possible worlds. The people of Rome are no longer certain of their destiny, and this uncertainty breeds anger, disunity, and occasionally violence in the streets.

In the meantime, the explorers have found several cul-de-sac worlds from which it is no longer possible to travel in a certain direction. Marcus visits one of these worlds, Roma DCX, in which two rival Christian Pope-Emperors rule separate empires from Rome and Constantinople. While walking around Londinium DCX, Marcus gets lost in the tubulum, an network of tunnels underground with a map that is not drawn to scale. This inspires him to think of the map of the Roman worlds from a different perspective, and by doing so, he comes up with a different direction in which to travel. He does so, and finds himself in an abandoned room in a completely unfamiliar version of Londinium. He soon realises that the city is at war, and although he can hear a man speaking on the shiner, vowing that his people will never surrender, the tone of the speech indicates that his side is losing the battle. And when Marcus tries to return to Londinium DCX, he finds that his bangle has stopped working.

By now, Marcus has a great deal of experience in travelling to unfamiliar worlds, and after stealing some clothing from an abandoned flat, he is able to pass unnoticed in the streets. However, this city of “London” is very different from the Londiniums he’s used to, and even when his bangle starts working again, he finds that he can only travel to different versions of London and not back to the familiar Roman worlds. He does gather that these versions of London are engaged in a battle with Germans, or “Nazis.” Unprepared for the speed of the traffic on the streets, he is hit by a car while trying to cross the road, and is taken to hospital -- where one of the nurses is his wife Angela, or rather a version of Angela who was born in this world. Marcus becomes convinced that she is still the woman he knows as his wife, and Angela also finds herself attracted to him for reasons she can’t explain. After Marcus is discharged, he asks Angela out for a drink, and she ends up inviting him back to her flat for the night.

The next day, Angela returns to the hospital while Marcus visits a library to research this world’s history. He soon realises that in these worlds, Rome fell to the barbarians, and history after its fall seems to him nothing but a parade of wars, famines and plagues. He then hears rumours that the Nazis have landed on British soil, and returns to Angela’s flat just as a German bomb falls in the backyard, obliterating the house and killing Angela’s landlady. Fortunately, Angela had not yet returned, but Marcus warns her that London is not prepared to fight off an invasion and the British generals must know this; from what he’s seen, it appears that they’ve withdrawn their soldiers, sacrificing the city rather than fight a battle they cannot win. Angela accepts this, but decides to return to her parents’ and help to defend their home.

Rather than retreat from the city to get an overview of the fighting, Marcus finds that he cannot abandon Angela; thus, he changes into full battle armour and follows her to her parents’ home. German paratroopers have landed in the city, and Marcus finds Angela and her father fighting an SS soldier in their street. Marcus easily kills the Nazi soldier, and admits the truth of his origins to the bewildered Angela. However, more Germans arrive and overpower Marcus, and the Understurmfuhrer in command of the squad scoffs at Marcus’ Roman armour and stabs him with his own gladius. Angela holds Marcus as he collapses, and he uses his bangle to transport them back through this cluster of worlds and across the great gap to Roma DCX.

While Marcus recovers from his injuries, Angela comes to terms with the reality of the Roman worlds; she finds them to be a utopia compared to her own world, but is rather shocked to learn that her counterpart was only 12 years old when she and Marcus married. After a month of recovery, Marcus is well enough to return to Roma I, where the two Angelas meet and quickly become closer than sisters. Marcus then visits the Emperor to request that the Roman army be assigned to invade the Nazi worlds and liberate their people from the barbarians. However, the discovery of other Romes has been causing unrest in Roma I, as people begin to question the principles of their utopia; a cult of ancestor worship has even sprung up involving the use of bone masks to celebrate death and change, which is against everything Rome stands for. The Emperor, fearing the consequences should people learn that there are histories in which Rome does not prevail, orders that all evidence of the other cluster’s existence is to be suppressed. Marcus publicly agrees to abide by the Emperor’s decision, but privately, he and his close associates plan to use the bangles to liberate the Nazi cluster themselves.

Marcus and his associates thus return to the Nazi worlds and begin their campaign, learning from their mistakes and using them to refine their tactics on subsequent worlds. However, on Germania LXI, they are ambushed while leading a routine attack on a German oil refinery, as if the Nazis knew they were coming -- and Marcus barely escapes from the man leading the SS troops, a tall, blond man with sharp, pointed teeth who appears to be looking specifically for him. When he discusses the incident with Don Vulpinus, Marcus realises that, while the distribution of Roman worlds seems entirely random, the Nazis’ grip on their territories seems tighter the deeper the Romans penetrate into the cluster. It has occurred to Don Vulpinus that, just as Roma I is an ideal version of history in which everything fell in Rome’s favour, so there must be an “ideal” Nazi history; and the presence of a pattern seems to imply that that the Nazis from the “central” version of history have developed the ability to travel between worlds. If this is so, then it may be possible for the Nazis to cross the Great Divide and attack the Roman cluster.

Marcus leaves Don Vulpinus in charge of the Romans’ campaign while he ventures deeper into the Nazi cluster, searching for Germania I. On the way, he encounters a world in which the Nazis have forced Britain’s surrender by using a weapon capable of unleashing the power of the atom; however, when he tries to return and warn his allies, he finds that his bangle will now only allow him to travel in a single direction. He is forced to continue onwards, and in the course of his travels he meets a version of Adolf Hitler who ruled a communist rather than a fascist dictatorship. This version of Hitler was deposed by a man he believes to be an impostor, who transformed his communist “utopia” into a fascist regime almost overnight, assisted by a blond man with pointed teeth named Herr Abschrift. Marcus is now certain that the Nazi worlds are interacting and that the technology has been provided by the mysterious Abschrift. He kills the Communist Hitler before continuing on his way, convinced that, just as the two Angelas are the same woman, so all the Hitlers are the same evil man.

Marcus finally reaches Berlin in Germania I, which is easily recognisable as the world in which everything has gone in the Nazis’ favour. Before he can explore, however, he is captured by soldiers led by Herr Abschrift, who causes Marcus’ bangle to heat up and melt away without even a gesture. Abschrift informs Marcus that he’s intervened in a War beyond his understanding and has provoked a response, and takes Marcus to the Senate, a gigantic domed building at the heart of Berlin. Each of the Senators is a version of Adolf Hitler from one of the Nazi worlds, led by the Hitler of Germania I. Abschrift has provided them with the technology to pool their resources, and once all of the Nazi worlds have been united into a single empire, they will be able to turn their attention to the Roman cluster and prevent the Romans from interfering in Abschrift’s War.

Marcus is held prisoner by the Nazis for the next sixteen years, tortured and interrogated and finally left to rot in prison. Fortunately, most of the answers he gives are born out of delirium, and there are great changes in the Roman cluster in his absence, so the Nazis are not as fully prepared as they believe when the time comes for them to attack. During his time in prison, Marcus gradually pieces together a picture of the War that Abschrift is fighting. Abschrift claims to be from the one true version of history, and seems to regard the Roman and Nazi worlds as somehow less real than his own. His history is protected by several Great Houses, who are currently at war with a terrible enemy. Abschrift serves House Mirraflex, and was sent to recover the old man and his twelve “friends,” who were apparently trying to escape from the War. Instead, he found the Romans using the old man’s time bangles to develop their Empire of Empires, and is now using the Nazi cluster to stop them in case the Empire of Empires falls into the hands of the Great Houses’ enemy.

With Abschrift’s help, the Nazis go on to conquer or dominate every world within their cluster. On 30 August 1945, Eva Braun of Germania I gives birth to Adolf Hitler’s son and heir, August Hitler. By 1951, the Nazis are ready to turn their attention to the Roman cluster. Nazi agents infiltrate Roma DCX and convince the Emperors of both Rome and Constantinople to construct huge arches to celebrate their achievements. The arches are in fact dimensional gateways that enable Nazi squadrons to storm through en masse, and the armies of Rome and Constantinople, each on guard against an attack from their traditional enemy, are caught entirely off guard by the attack from within. Within ten days, the Nazis are in control of Roma DCX.

Those who control the Roman time bangles soon discover what has happened. The Romans of DCX are still fighting resistance against the Nazi invaders, and while the Nazis struggle to keep their foothold in that history, the military planners of Roma I track down and capture Nazi agents in the other histories. The Romans then study the Nazis’ technology and use it to improve their own ability to travel between worlds. The Emperor of Roma I dies of old age, and his successor, Eugenius, stops suppressing news of the Nazi worlds and allows the problem to be addressed. Two of Marcus’ allies, Terrance Ollacondire and Regina, discover through the use of adapted Nazi technology that there are beings other than Nazis or Romans travelling between the worlds, and they follow one such trail to Roma XIII, a world obsessed with death and ancestor worship. Here, they meet Consobrinus Patruelis, a member of Faction Paradox, who takes an interest in the Romans and offers to help them construct temples to the goddess of Time, Clepsydra Metiri. Within these temples, with the right rituals and ceremonies, time and space can be folded into new shapes, and the Romans are able to construct a fleet of time-travelling warships with which to take back Roma DCX and move on to eradicate the Nazis.

Sixteen years after his capture, Marcus is removed from Spandau and taken to the version of London bordering on Roma DCX. Just outside the Nazi keep that was once Windsor Castle, his escort is ambushed by Roman soldiers who are scouting out the area before their invasion. Rather than flee with the Romans, Marcus allows himself to be recaptured by the Nazis but smuggles a small shiner into the Castle with him. Inside, he is taken to a reception at which the aging Adolf Hitler of this world addresses a group of high-ranking Nazi officials, announcing preparations for a blitzkrieg invasion of the Roman worlds. Marcus transmits the speech with his shiner, and the Romans, believing themselves forewarned, use their new technology to materialise in the middle of occupied London and seize the city. But Marcus discovers too late that August Hitler, now a teenager and a loyal Nazi, manipulated him in order to lead the Romans into a trap; the Nazi archway in the centre of London is also an atomic bomb, and Adolf Hitler now triggers it, destroying London and wiping out nineteen of the twenty Roman legions who had invaded. The twentieth legion, led by Marcus’ brother Titus, is already attacking Windsor Castle to rescue Marcus; in the confusion, Marcus kills this world’s version of Adolf Hitler, but August vanishes into thin air, escaping to another Nazi world. Marcus vows to kill August Hitler personally.

The Roman army has suffered the single most devastating defeat in its history. The new Emperor wishes to turn the same weapons against the Nazis, but when the scouts sent to the ruins of London die of cancer and internal bleeding within weeks -- including the Emperor’s brother-in-law -- he concedes that this is an irredeemably evil weapon and pledges his support to fight the Nazi scourge. The best legions of the best Romes are brought together into one massive fighting force, and the war between the Romans and Nazis begins in earnest. Marcus directs operations from the London in which he met the alternative Angela. The Romans begin each attack on a Nazi world by destroying the gateway that enables the Nazis to send in reinforcements, and then attacking the parade grounds at Nuremberg and broadcasting their victory to the world, inspiring those conquered by the Nazis to rise up against them. Not all of the liberated people are happy to see the Romans, believing that they’ve exchanged one dictatorship for another -- but they’re even less happy with the Nazis, who thus find no allies when they try to retake the conquered worlds.

Over the next several years, the Romans drive the Nazis back towards Germania I, but the fighting becomes fiercer as the Germans consolidate their position. In the year 1964, someone releases cyanide gas into the Senate of Hitlers just after they pass a resolution declaring that August Hitler shall become Fuhrer when the last of the Adolfs has died. August blames the Romans for the attack, but the attack means that he has now become Fuhrer himself, and his first act is to grant himself sweeping emergency powers. Consobrinus Patruelis helps the Romans to seal off the Divide, preventing the Nazis from crossing over to attack the Roman cluster. Marcus’ brother Titus dies in a random accident, but by this time the war has consumed Marcus and he finds that he is unable to grieve.

Finally, in the year 1970, the Nazis are driven back to Germania I, and Marcus takes personal command of the Roman battalions as they advance on the Rhine. After a brutal battle, the Romans defeat the Germans and drive them back to Berlin. Every German in the city is a loyal member of the Nazi party, and Marcus’ old friends Regina and Quintus Saxus are killed in the fighting, but the Romans have superior tactics and numbers, and on 10 May, General Wilding, the commander of the German troops, surrenders to the Romans. August Hitler and Abschrift have vanished, but within a month, Abschrift is spotted and captured near the Tramontane Gate, the single gateway that connects the Roman and barbarian clusters. Abschrift reveals that, since he failed to contain the Roman threat, he’s summoned the Great Houses to deal with them -- and they are coming in person to destroy all of these worlds, to prevent a united cluster of two thousand histories from falling into the hands of their enemy.

Marcus sends Terence Ollacondire back to the Roman cluster with instructions to seal the gate; he knows that he will never see Angela again, but remains on the other side nonetheless in case of trouble. One of Abschrift’s masters arrives with a device that will destroy the entire Roman cluster, but before Abschrift can use the disc, Marcus kills him. The godlike time-traveller brings several copies of Abschrift back to life from the original’s blood, but Marcus holds them off until the Romans have done their work and sealed their cluster from the inside. He then pleads with the time-traveller to spare the barbarian worlds as well, and the god agrees on condition that Marcus remain within the cluster and police it to ensure that these worlds do not end up posing a threat to the Great Houses.

Marcus accepts these terms, and spends the next decade of his life patrolling the barbarian cluster and helping to restore civilisation to the worlds devastated by the war. However, he remembers his vow to kill August Hitler, who was allowed to escape to the “True Earth” by the time-travellers for services rendered to Abschrift. With the help of allies whom he refuses to identify, Marcus follows August to Earth and spends another nine years tracking him down to a small villa in Brazil. By now, Marcus is old and weary of war, and he simply kills August by crushing his throat. His work done, Marcus writes his memoirs, satisfied that the Nazi scourge has been eliminated and that, even in the “True Earth,” where Rome fell centuries ago, the memory of its glory prevails.