Sword at Sunset, by Rosemary Sutcliff, has been created by the author by doing extensive research into the Arthurian legends. Her research took her to some strikingly similar accounts of a warrior king named Artos, the Bear. It is from this research that the author has crafted this pre-arthur tale. Readers will note that much of the basic Arthur tale is very much intact, lending credibility to the fact that this may have been the real person that the legend was based on.

The tale begins with a very old warrior lying on his bed. He is gravely wounded and ill. As he drifts in and out of consciousness, he begins to flash back to his life and exploits. The reader learns that this warrior has kept his promise to his adopted father, to keep Britain united. As the first section ends, the reader learns that this wounded warrior, is, in fact, Artos, High King of Britain.

The next section begins at the beginning of Artos's adult life. He is a young man, in his early twenties, and living in King Ambrosius's court. His father, Utha, was the brother of the King, and had gotten a country wench pregnant 20 years previously. Unlike many nobles of the time, Utha had taken Artos in when the boy's mother died. Though he does not acknowledge him as anything other than his bastard, it has become clear to everyone at court, that noble blood runs in Artos's veins. Utha, the reader learns, has died in a hunting accident many years prior, when Artos was in his pre-teen years. Ambrosius saw to his upbringing since then, and the two have become closer than Artos had ever been to his real father. The reader is told that his name, Artos, is the Roman version of the name, Arthur.

Since Artos has come of age, Ambrosius has trained him to the sword and the horse, and Artos has excelled at it. He has a way about him that draws men to his side. He straddles two worlds, the reader learns. His mother was Pictish, one of the 'dark people', while his father was a Britain. While he was raised in Christianity, he also respects and honors the religion of his mother's people as well. People respect him for this, but the Catholic church considers him to be bordering on heretical.

Ambrosius's greatest concern is that the Sea Wolves, joined by the Scots, have been harrying some of the outer lying villages. It is Ambroisius's vision to not only bring all of the smaller fiefdoms into one large kingdom, but to raise it up into a greater whole. But the Sea Wolves threaten to undo this. He sends Artos to purchase war horses so that he can bolster his armies to go and confront the Sea Wolves when they next make landfall. Artos willingly agrees.

He and his men, foremost among them a large burly warrior named Cei, travel by boat to an area known for its horses. Once there, Artos begins looking for the horses that will make the foundation of Ambrosius's horse farm. He spies a large, fiery, black stallion, but seeing how wild it is, passes it by. There is a small man standing nearby that suggests that the black stallion would behave for the right person. Challenged, Artos asks him to prove it. The man, who gives his name as Bedwyr, grins and approaches the horse. True to his word, the black stallion calms and performs magnificently as the small man puts him through his paces. Artos is impressed but is still not convinced. Bedwyr states that he will personally come back with the horse, and if he and the horse arrive safely in Venta, then Artos must be bound to keep both him and the stallion. Artos laughs at the boldness of the offer, and accepts. Both the black stallion and Bedwyr make it back in one piece. Bedwyr is also a minstrel and his music is most welcome.

Ambrosius gives the war horse development program over to Artos entirely, something which he takes to readily. Word arrives that the Sea Wolves have made landfall once again, and this time Ambrosius sends Artos out with a band of his chosen companions to keep them at bay. Artos is so successful that news of his victory reaches Ambrosius before Artos does. Ambrosius calls him into his chambers and presents him with one of the royal gems with the seal of Britain on it. Artos, at first, refuses stating that he is just a bastard, even if half of him is noble. Ambrosius tells him that this particular seal is not the official one and that his father would have wanted him to have it. He also gives him the royal armband, declaring him the Prince of Britain. Artos is overwhelmed by the gift and immediately has it set into the pommel of his sword.

Ambrosius begins sending Artos on numerous campaigns and each time Artos's legend grows. Companions come out of the hills and hollows to meet with him and to join him. He and his men often stop at friendly villages on their way to and from the coast. When the weather turns bad, they take sanctuary in the hills of Badon. He takes refuge in the home of a woman that looks vaguely familiar to him, though she assures him that they have never met. She drugs him, seduces him, and in the morning, in horror, finds that this woman, named Ygerna, is his half sister. She is bitter that Utha should have favored him with a place at the court, but not her. She says that if a child comes of the union she will raise him, call him Medraut, and when he is a man will send him to Artos. Shaken, Artos flees, finds his men, and they make for the coast to continue their campaign.

Artos forgets the incident, though he now views all women as suspect. In fact, he cannot lie with another woman because of it. He makes it a rule for his inner circle of companions that they refrain from marrying, though they may seek comfort where they will. Families and wives will only complicate things, he suggests to them, and they will not have their full focus on the tasks at hand. They go to a neighboring fiefdom where Prince Cador has had a son, named Constantine. While Artos is viewing the child, the royal seal falls from his sword into the child's cradle. Artos acts as if it is nothing, but he is very superstitious and feels that it is an omen of some kind.

There are many campaigns, most of the brutal and bloody. After one skirmish, he and his companions find a destroyed village where a small pictish woman has been brutally savaged by the Saxons. They are all very appalled, and to honor the poor woman's spirit, Artos buries her according to her people's customs. Her people come for her body later in the evening, but finding that Artos has honored her, they again, pledge their loyalty to him. They give him the name Artos, the Bear. Another incident, after another brutal battle, leaves Artos and his men into a fortress that has been taken. There, in the inner sanctum, is the young prince, Cerdic, who is keeping guard over the body of his mother, who has died. The boy tries to fight them off, but Artos pins his arms back until he yields. Bedwyr urges Artos to kill the boy as he will only come back to haunt him later. Artos can't kill a child, and he sets the child free.

As they make their way back to Venta and the court of Ambrosius, they stop to celebrate a May Day festival. There Artos sees a woman that completely captures his attention. Her name is Guenahara (Guen). Her father, seeing that they are attracted to one another encourages the match, and cleverly maneuvers Artos into a situation where he may cause a political incident if he does not marry her. He and Guen discuss the situation and they decide to marry. This temporarily upsets Artos's men, but in the end, since she brought a large dowry, many horses, and many fighting men, they are satisfied that Artos has only done this to make their group stronger.

Guen will not be parted from Artos, though he tells her the war trail is no place for her. She is adamant, so he allows her to stay. Durim Dhu, the ambassador from the dark people, comes out of the hills in the fall to warn Artos that the coming winter would be extremely harsh and that he should move most of his horses further south. Artos agrees to do so, sending some of his men with them. Durim Dhu is correct and the winter nearly kills Artos, Guen, and the rest of the troops that had overwintered in a fortress half way to Venta. Eventually, several men volunteer to try and get through to Venta to have more supplies sent. One of them does make it through and supplies arrive just in the nick of time. It is during this time that Guen learns that she is pregnant.

The spring arrives and as soon as the snows disappear, they make leave to travel the rest of the way to Venta. By this time Guen is very pregnant, but is adamant that she will give birth to her child either in Venta, or under her father's roof. They stop for the night at Durim Dhu's village and Guen is horrified. She thinks that the people are fairies and will steal her child. This is only reinforced when she goes into early labor that night. The midwife delivers the child, a girl, whom they name Hylin. The midwife warns Artos that the child has been born under the wrong star and should be set out on the hills for the wolf people. Artos will not hear of it.

They make it back to Venta and Ambrosius calls for him. He tells Artos that he could not be more proud of him if he were his own son. Ambrosius does not have an heir, and he would like to make Artos his formal heir, but he explains that he can't do that because based on blood there are others in the kingdom that have a greater claim. The only way Artos will be king, which is what Ambrosius wants, is if the people are so supportive it would be madness to proclaim another in his place. He tells Artos how to accomplish this. Artos doesn't have long to consider all of this before the Sea Wolves land again. Once more, he must leave Venta. This time Guen stays behind with the child. Bedwyr, who has been injured, stays behind in her care as well, upon Artos's suggestion.

In this campaign, he meets the grown child, Cerdic, whose life he has spared so many years ago. Cerdic intends to kill Artos if he can. He fails, but Artos manages to kill his Uncle and leaders that had rallied to his cause. Cerdic flees, once again. Cei tells Artos that he should have pursued Cerdic, instead of still leaving him alive, but Artos has received word that Hylin is gravely ill. There is one more battle that must be fought, and he feels that if he leaves, his men will fall to the sword of the enemy. He waits until after the battle is won the next day to leave for Venta. When he arrives his daughter has died. Guen blames him for leaving them alone. She angrily tells Artos that in the end, all she did was cry for Bedwyr's harp playing.

It is as Artos is leaving the death chambers and crossing the courtyard that a young man approaches him from the shadows. He knows that this is the child that Ygerna told him would come. He gives his name as Medraut and Artos acknowledges him, though he does not have any feelings for the boy at all. Medraut states that he only asks to be made a part of Artos's companions. Artos agrees.

Meanwhile, King Ambrosius's health is failing. He has the wasting disease and knows that he won't be alive much longer. He calls Artos to his chambers and tells him that he wants one last hunt with his closest friends. He invites Artos, Aquilla, and several others to go with him to Spain to hunt stag. While there, Ambrosius jumps from his horse, wades in to deliver the killing blow to the stag, and is gored to death. Artos and the men knew that this was Ambrosius's plan all along, to choose the date and time of his own death, rather than dying a slow and agonizing death indoors.

Meanwhile, the Sea Wolves are back. Artos sets out with a large contingency to fight them once more. Bedwyr, however, has recovered, but left the castle to tend to his own lands. Artos misses his best friend, but does not begrudge him some time. Guen seems as distant as ever, and it has become clear to him that Medraut is a destroyer, whose enjoyment of killing has not gone unnoticed by Artos's inner circle. They warn Artos that he should put Medraut aside. Artos can't do that. Artos knows, thanks to Ambrosius's shrewd statescraft, that if he can win this largest battle with the Sea Wolves that the people will demand that he be king.

It is hard fought, but he does win. He is so excited by the victory that he urges the men to ride with him through the night so that they might be back at Venta before morning. They all begin calling him Caesar, and they go with him back to Venta. Unfortunately, when Artos arrives back early from the campaign, he finds Guen and Bedwyr together. He exiles them both, and is heartbroken.

Meanwhile, the council convenes and wonders if Artos should not be the next High King. With his decisive victory, he is crowned within the week. Artos tries to turn his mind and his heart to leading the fledgling country, but he is heartsore. His betrayal is twofold where Guen and Bedwyr are concerned, and he doesn't seem to be able to focus. He receives word that Cedric has returned with reinforcements and that at this time the Scots have regrouped to join them. He also learns that Medraut has betrayed him and joined forces with Cerdic against him. Though he is ill, he mounts his horse, gathers his men, and sets out to meet the Sea Wolves.

While he is camped, on the eve of the battle, his tent flap is pulled back and a man enters. It is Bedwyr. Artos forgives everything and is glad to see him. Artos learns that Bewyr and Guen did not live happily ever after, and for his part in hurting Artos, he is sorry. They discuss Merdraut, and Artos says that his one and only goal for the battle the next day, aside from victory, is to meet Medraut and put an end to the curse once and for all. Bedwyr agrees.

The next day Artos and Medraut do face one another. Artos knocks Medraut from his horse, into the water. While Artos is trying to figure out where Medraut has gone, Medraut surges up from the water and runs his sword through Artos's hip, leg, and into his chest. Artos reacts by slitting Medraut's throat. As the boy slips into the water, Artos wonders that Medraut had not known that the only way the curse would be lifted would be if they both died at one another's hands. Everything goes black.

Artos wakes in strange chambers. The first face he sees is Bedwyr's. Artos is surprised that he is not dead yet. Bedwyr tells him that they have brought him to the island of apples, a type of monastery. Artos tells him not to tell anyone of his death, and to send for Constantine, Cador's son, who is now a man. Constantine arrives and Artos names him his heir to the throne. He tells them to shroud his disappearance in mystery, so that Britain might remain united, and that his men might continue to fight as if he were still commanding them from a distance. Everyone agrees. Artos dies.