The Trojan Aeneas fled with his son Ascanius and conquered Italy, and when Ascanius' son Brutus was kicked out for accidentaly killing his dad in a hunting accident he ended up in Greece where he liberated the descendants of all the other Trojans who hadn't escaped and had been enslaved by their conquerors. Guided by the Goddess Diana the disposessed Trojans set to sea and all ended up in a remote land "once occupied by giants", which they named (slightly) after their leader - Britain.
The new inhabitants settled the islands of Britain and Brutus' pagan dynasty ruled for centuries, including King Leir (as made famous by Shakespeare), until in AD 43 the islands were conquered by The Romans. Fortunately their sordid empire collapsed a few centuries later in AD 418
Unfortunately this left a civilised, weakened Christian Britain ill-prepared to face the onslaught of invading barbarian pagan tribes of Irish, Scots, Picts, Norwegians and Danes.
One day two Saxon brothers called Hengist and Horsa landed with their warriors and families in three longships, and Vortigern gave them lands to settle in exchange for their help fighting against the invading Picts. This policy was initially so successful that they were soon granted permission to bring over eighteen boatloads more of their countrymen, including Hengist's own lissom daughter Renwein, whom Vortigern was wassailing in no time in return for the kingdom of Kent. Hundreds more ships followed.
The Saxons were now so numerous in Britain that the native British people feared them, and so they deposed Vortigern and raised up his son Vortimer instead. He successfully drove the Saxons off the island, until his stepmom Renwein poisoned him. Then Vortigern was made king again, and ordered a peace conference between the British and the Saxons to take place on Beltayne at the Cloister of Ambrius. Treacherously the Saxons hid daggers under their robes, and at a pre-arranged signal fatally stabbed nearly every Briton present. Vortigern was captured, forced to hand over the entire country to the Saxons, and fled to Wales.
Merlin now pulled off the first of what was to become a career of neat tricks. He challenged the magicians who had recommended his blood-letting to explain why the tower of Vortigern could not be built. When they had to admit that they could not, he gave his own explanation. This was proven when according to his instructions the workmen dug deep below the site and found a pool, then drained the pool and found two hollow stones, and in each was curled up a sleeping dragon; one white, one red. These awoke and began to fight one another furiously, and Merlin explained that these were the opposing forces of the (white) Saxons and the (red) Celts. He followed up with several pages of alarming prophesies over a thousand years before Nostradamus and often just as baffling. Here are some free samples:
Britain's mountains and valleys shall be levelled,
and the streams in its valleys shall run with blood.
The cult of religion shall be destroyed completely
and the ruin of the churches shall be clear for all to see.
Seven who hold the sceptre shall be perish
one of them being canonised.
A king who is blessed will fit out a navy
and will be reckoned the twelfth in the court
among the saints.
Those who have had their hair waved shall dress in woollen stuffs of many colours,
and the outer garments shall be a fair index of the thoughts within.
The feet of those that bark shall be cut off.
Intent on building a particularly splendid memorial to the British dead, Aurelius accepted the advice of Archbishop Tremorinus and consulted Merlin. On his advice they invaded Ireland and stole The Giant's Ring, which Merlin re-erected over the burial mound on Mount Ambrius, thereby creating either Stonehenge or Avebury. Aurelius himself was crowned, and all seemed well.
Meanwhile Vortigern's son Paschent had raised yet another army in Germany and attacked the North of Britain. Aurelius quickly chased him off but he took refuge with the still-smarting Irish, who helped him re-invade almost immediately. Aurelius fell ill so Uther led the British armies, but Paschent sent a Saxon called Eopa to infiltrate Aurelius' court disguised as a British doctor, and he fatally poisoned the king.
Having finally made Britain safe Uther called for a huge party to celebrate, but Gorlois turned up with his utter babe of a wife Ygerna and Uther fell instantly in lust, so Gorlois and his girl got up and left. Determined to have her at any cost the lust-crazed Uther invaded Cornwall and besieged Gorlois at Camp Dimilioc and Ygerna at Tintagel Castle, but his sieges failed.
Desperate, Uther persuaded Merlin to supply magical drugs to himself, Ulfin of Ridcaradoch and Merlin in order to change their appearances into those of Gorlois and his companions Jordan of Tintagel and Britaelis respectively, and in these forms they entered Tintagel Castle, so that Ygerna believed she was merely doing her wifely duty on a conjugal visit. Meanwhile the real Gorlois had personally led a foolhardy attack on the forces besieging him at Camp Dimilioc and been killed, so in the end Uther won the war and married Ygerna as himself anyway.
Merlin vanishes from Monmouth's narrative after this, and so never meets Arthur.
Many years later Uther fell ill and Octa and Eosa escaped from prison, raised an army in Germany and returned to ravish Britain until Uther, unable to walk and borne in a litter, attacked and slaughtered them all at The Battle of St Albans.
But then the treacherous Saxons poisoned his well, and Uther died and was buried alongside his father in the Giant's Ring.
By the time of his father Uther's death Arthur was fifteen, and accepted the leadership of the British people against the Saxons.
He first defeated the German Colgrin at The Battle of the River Douglas, forcing him to flee to York, where Arthur besieged him until Colgrin's brother Baldulf and Duke Cheldric arrived from Germany and chased Arthur back to London.
Arthur then returned to massacring the various invading pagan Scots, Picts and Irish, and rebuilding churches.
Then, bored, he set off to conquer the rest of Europe, starting with Norway (which he gave to Loth), then burning and pillaging Denmark and Gaul.
Within nine years all Europe was his, so he gave Neustria (Normandy) to his cup-bearer Bedwyr, and Anjou to his Seneschal Kay, and was home for spring.
Meanwhile Lucius assembled his own vast force from Rome's client kingdoms, including
Arthur left his queen Ganhumara and his nephew Modred in charge of Britain and sailed from Southampton, having unsettling dreams about dragons frying flying bears on the way.
Pausing only to personally slay an evil giant that had just frightened Duke Hoel's niece Helena to death and ravished her nurse, Arthur marshalled his troops at Autun and sent Boso of Rydychen (AKA Oxford), Gerin of Chartres and his nephew Gawain to meet Lucius to give him a chance to surrender. Unfortunately Gawain lost his temper and decapitated Lucius' nephew, then accidentaly led their Roman pursuers into a handy spontaneous ambush by the British where they were slaughtered and many taken prisoner.
Arthur was delighted, and sent the prisoners under escort to Paris. The Romans attacked the convoy but were again decisively beaten, though this time with heavy casualties on both sides.
Rattled, Lucius decided to shelter his army in the city of Langres, intent on then moving on to the safer haven of the city of Autun where he could await reinforcements from Emperor Leo, but Arthur besieged him there until he came out to fight.
Bedwyr tragically died and Kay was mortally wounded in the first massive charge, but Arthur's nephews Duke Hoel and Gawain heroically succeeded in hacking their way through the Emperor's bodyguard and up to Lucius himself, where Gawain engaged him in single combat. The Romans almost recovered and drove them back, but then Arthur arrived in a berserk kill-frenzy, and soon the battle achieved wholly new and undreamed-of levels of hackage and maimage.
Then Morvid, Earl of Gloucester broke from cover high in the hills where Arthur had posted him, and attacking the unsuspecting Romans in the rear scattered them and slew Lucius. The Britons pursued the fleeing Romans and slaughtered them without mercy, and Arthur despatched the corpse of Lucius back to the Roman senate, with a message stating that this was the only tribute they would ever receive from Britain.
Arthur buried the dead and Wintered his troops there, and next Summer made ready to attack Rome itself.
He retreated to Winchester, and when Ganhumara learned of Arthur's victory she fled from York to Caerleon and became a nun.
There was much hideous slaughter on both sides, even after Arthur and his own division of 6,666 men hacked their way through the enemy lines and slew Modred. Although Arthur's army was (just) victorious, he himself was badly wounded and was carried off to The Isle of Avalon so that his wounds could be healed.
This is the last mention of King Arthur. Although Arthur is not actually said to have died, he never appears again in Monmouth's narrative.
He eventually cornered and slew one of them beside the alter in the church of Saint Amphibalus, then the other in a London monastery. For this sacrilege he was struck down by the (un-specified) vengeance of God, and was buried alongside Constantine II and Uther in the Giant's Ring.
Eventually, several kings later, the British got their arses kicked. The last British king Cadwallader died in AD 689, and although his two sons Yvor and Yni somehow harassed the English for 79 years, the Saxon advance was unstoppable.
The British were steadily pushed Westwards and were given the name Welsh instead of British. The Saxons, Danes, Jutes and Angles became the English, ruling the greater part of a land they renamed England, or Loegria.
The first English king was called Adelstan, and he "ruled over the whole of Loegria">
Similarly, in different parts of the text she is described as being married not to Budicius, but to King Loth of Lothian, bearing the brothers Modred and Gawain, who are therefore Arthur's nephews. Elsewhere, Loth is described as being married to a nameless sister, not of Arthur, but of Arthur's father Uther, making Modred and Gawain Arthur's cousins. They are described both ways in the text. Did Monmouth write the whole thing in one go without checking it ?
Further confusion is caused by Monmouth's description of Constantine III as being Arthur's cousin. For this to be true his father Duke Cador of Cornwall would have had to have married Arthur's nameless auntie (Uther's sister), which either means Loth married someone else (maybe Arthur's second younger sister by Uther out of Ygerna) or Uther had two sisters.
Lugodoc's best efforts to hammer the kinks out of Monmouth look like this:
This version explains