The Wicked Day is the story of Mordred’s life. Mordred is cared for by Sula and Brude whom he believes to be his parents. Brude is a fisherman and Mordred grows up learning the trade. He is accustomed to hard work. When he is ten years old, he saves Gawain, King Lot and Queen Morgause’s oldest son, when he hurts his ankle while climbing a cliff. With King Lot dead and rumors of Merlin’s death, Morgause takes this opportunity to invite Mordred to her court. She believes it is safe for him to come out of hiding. She does not tell him who he is, but lets him believe that he is King Lot’s illegitimate son. She has Sula and Brude killed so that they cannot tell him the truth of his parentage and so that he has to stay in the court.
Morgause’s children by King Lot, Gawain, Agravain, Gaheris and Gareth have mixed feelings about Mordred joining their ranks. Gareth is just a baby so does not think much of it as he spends much of his time with Morgause. The other children see him as another obstacle to their mother’s affection. Mordred is treated as one of the family and absorbed into the palace life.
One day a ship brings an envoy from King Arthur to the Orkney Islands. Morgause and her five sons are to travel to Camelot and the envoy will stay in Orkney and take over her Kingdom for her. Morgause is worried what this summons might mean. She is worried for her sons. The boys are excited to go to Camelot and meet the High King. Morgause attempts to find out if Mordred has any magical powers before they leave for Camelot. She finds him severely lacking.
On their way to Camelot, Morgause and her sons stop to see her sister, Queen Morgan in Luguvallium. Morgan is in seclusion for having her lover steal Arthur’s sword. Arthur killed the man and Nimue, King Arthur’s enchantress, is in Luguvallium after having retrieved the sword. Morgan tells Morgause that she also has been summoned by Arthur. Mordred hears part of a conversation between Morgause and Morgan talking about him and why Arthur has summoned him. Morgan believes it is to kill him. Mordred attempts to runaway to avoid his fate. Nimue sensing something tells the King to put a full guard on the castle to keep everyone in. Mordred must return to his room in defeat. The guards follow him closely the rest of the journey to Camelot.
When they reach Summers County, Morgause refuses to spend the night at the castle of King Melwas, but insists on going to Camelot. The guards at the gates of Camelot make her wait by the gate as word is sent to King Arthur. He comes to the gate to meet her party and takes Morgause aside. While he is talking to her a messenger comes to tell him the Merlin is alive. He gives orders for Morgause to be kept away from anyone at court and rides off to meet up with Merlin.
The boys are soon taken to Caerleon for Christmas. King Arthur goes straight there after meeting up with Merlin. The boys begin training with the King’s companions to learn how to fight and ride. Morgause is taken to the nunnery at Amesbury and in the Spring will go north to Caer Eidyn where her sister Morgan already resides.
Arthur procrastinates about what to do with the boys. His hand is forced when Mordred finding a talisman of Sula’s on Gabran, Morgause’s former lover, accuses the man of murdering Sula and Brude and kills him. The guards take Mordred straight to the High King. Mordred is terrified. After hearing his story, Arthur tells Mordred that Queen Morgause is his mother and that he is his father. He tells Mordred that his mother planned to use him as a pawn to gain power. Arthur will keep Mordred in his court with the other boys as his nephew. He cannot openly call him son because of the circumstances and because of Queen Guinevere. Arthur will tell Gawain the truth so that he does not see Mordred as a threat to his Kingdom, but no one else.
Gawain keeps a guarded friendship with Mordred knowing that he could one day be High King. Agravain becomes more accepting of Mordred because he sees killing as proof of manhood and Gaheris sees it as avenges for all of them. He did not like Gabran’s relationship with his mother. The boys go to say their farewells to their mother as she is about to be moved to another nunnery. They are drawn back under her spell and Morgause finds a new admirer in Lamorak, one of Arthur’s Companions. In the Convent, Mordred can tell that Morgause has taken furniture from her rooms to make it look like she does not have the privileges her station deserves. It is an appeal to feel sorry for her. Mordred tells her that Arthur told him of his parentage. He stands up to her and does not show her fear or respect. Morgause tells Mordred that she kept him alive because Merlin prophesized that he would bring doom on Arthur and this will be her revenge against Merlin and Arthur. Mordred vows that he will not betray his King.
Morgan is moved south to a Castle that Arthur gave her in happier times among the hills of Caerleon. She is kept prisoner in the Castle with women who were willing to stay with her in captivity. Nimue advices Arthur to keep both of his sisters in captivity, but to let their keep their stations and luxuries. He cannot make them martyrs. Morgause is allowed to stay at Amesbury instead of being moved North. Over the years, Morgause comes to realize that she will never leave Amesbury so she uses her magic to keep her beauty and takes lovers. Lamorak is taken as her lover. Gaheris coming to visit his mother finds her in bed with Lamorak. Mordred witnesses him kill Morgause striking her neck with his sword several times. Mordred gives Lamorak his clothes and tells him to go. Lamorak tells Mordred that he was to marry Morgause.
Gaheris vows to kill Lamorak for the shame upon his family as he killed his mother for causing them shame. Mordred makes him leave the Convent before anyone finds him there. He will head north to escape the punishment of the King. Gaheris is banished from the Kingdom and Lamorak is told to make himself absent until the dust settles. Gawain swears vengeance on both Lamorak and his brother. Morgause’s death makes people see Mordred as “Prince Mordred” and known to the close to the High King and the Queen in love and favor.
Rumors of a relationship between Bedwyr and Queen Guinevere begin to circulate around the Kingdom. Due to Bedwyr’s close relationship with the King no one takes heed. The young sons of Morgause are spoiling for battle. Agravain is not happy that Arthur talks peace with the Saxons rather than run them out of Britain. The young Companions have not seen battle in this peaceful Kingdom and do not feel like true men.
Mordred goes to see Nimue to question her about the prophecy. She tells him that Merlin protected him because he knew that out of evil good could come. He also saw that in the moment of Arthur’s death his glory would be starting to pass and it would be time for his reign to end. She tells him that he cannot avert his fate. On the diplomatic trip with the King, Mordred hears talk from the young soldiers in Arthur’s company, the Young Celts as they are called, who are friends with Agravain. They are talking about Bedwyr and the Queen. They do not like the peace with the Saxons or the continuous talk around the Round Table. They want action.
Arthur asks Mordred to keep an eye on Agravain. Lamorak is returning to Britain and going to Northumbria. Mordred informs him that Agravain is in touch with Gaheris and he believes he is in Northumbria. Arthur devises the plan to bring the brothers all back to Camelot. He will send Agravain and Mordred to escort Gaheris. Agravain and Mordred find Gaheris living in a tower in the North with a young pregnant woman. He treats her as a servant. Agravain and Gaheris trick Mordred and ride out in the morning to ambush Lamorak who Gaheris knows will be heading to Northumbria. Mordred races to catch up to them, but is too late to stop their killing of Lamorak. Laboral’s friend, Drustan, appears with Gareth and some soldiers on their way to meet Lamorak to escort him to their castle. Gareth tries to talk to Gaheris, but he and Agravain are challenging Drustan to fight. Drustan will not fight them because of orders of the High King. As Drustan rides away, Agravain and Gareth attempt to talk sense into Gaheris. Gaheris pushes them away and goes after Drustan. Agravain after a moment’s hesitation follows him. Agravain is hurt and he and Gaheris are taken into custody by Drustan.
Mordred who stayed hidden in the woods during this altercation begins the journey back to Caerleon alone. He stops by Merlin’s cave to see him about the prophecy, but no one is there. When Mordred reaches Caerleon, the King has heard of Lamorak’s death and was very angry. Gareth pleaded successfully for his brothers’ pardons. Bedwyr does not like the Orkney brothers and tries to persuade the King not to the brother’s return to court.
Mordred insinuates himself in the Young Celt’s group to keep watch on them for the King. The group hate Bedwyr and want to shut him out of the King’s favor. They see his relationship with Queen Guinevere as their way. Arthur tells Mordred that he is planning a trip to Brittany and wants Mordred to stay in Camelot and be Bedwyr’s deputy. Mordred asks the King if he could take Bedwyr with him for he fears the actions of the Young Celts. Arthur tells him that there are things best not spoken of or known.
One night while the King is away, Bedwyr receives a summons from the Queen. She has a letter she wants him to see. The young companions lead by Gaheris and Agravain see this as their opportunity to expose Bedwyr. They crash the Queen quarters only to find Bedwyr, the Queen and one of her woman in the bedchamber talking. They young men attack Bedwyr with Gareth and Mordred trying to stop the fighting. Agravain and Gareth are killed. Gaheris goes after the Queen and Mordred saves her suffering wounds. Gaheris receives slight wounds. Gaheris uses Mordred to escape Camelot and leaves him with a community of shepherds to heal his wounds.
Gawain, now King of the Orkneys, finds Mordred on Orkney working the land as his father, Brude had. He has a son, which he sired before leaving Orkney working with him. Gawain tells him that it was assumed that he was dead. Gaheris died of his wounds, which he did not bother to attend so no one knew what happened to Mordred. They King sent men looking for him. The Queen told him that Mordred attempted to save her from Gaheris and was not part of the attack. Mordred leaves the Orkney Islands and returns to the King’s court.
King Arthur and Queen Guinevere receive Mordred with pleasure. Bedwyr has gone to Brittany to his castle of Benoic. He has married. There is also trouble in Briittany. The new Roman Emperor Justinian wants to restore the glory of the Roman Empire. He has asked for tribute from Brittany and the other surrounding lands. Arthur fears that he will also set his sights on Britain. The King and Mordred travel to Brittany to see King Hoel. Hoel and Arthur discuss sending an embassy to Consul Hiberus to discuss his proposals, but really to see their strength. Arthur cannot go himself so he chooses Gawain to go. He sends Mordred back to Britain to be regent in his place. Duke Constantine, Arthur’s heir, is attempting to take control of the Kingdom in the King’s absence. Arthur is sending Mordred to rule alongside Queen Guinevere to stop Constantine from taking over. Arthur sees Mordred as his heir and gives him instructions for the Kingdom in case of his death. Mordred is to seek an alliance with Cerdic, the Saxon, to ensure the Romans do not take Britain.
Gawain is hot headed and starts a fight during the talks with the Romans. It ends with the men having to run for their lives. They are able to fight the Romans back, but his actions cause a war. Arthur is believed to be lost in the ensuing battle, but only went with some troops to hunt down escaping Roman soldiers. He returns to camp after the message has gone to Britain of his death. Mordred puts the plans that he and King Arthur talked about into action. He speaks to Cerdic and forms an alliance. He also makes alliances with Kings in the North. These actions are seen as treasonous by Duke Constantine who writes to Arthur.
King Arthur returns to Britain with Gawain and his troops. Their ships are caught in a storm and thrown off course. Their standards are shredded in the storm so that when the ships crash upon the Saxon shores they are suspicious that the ships do not have standards. This leads them to attack King Arthur and his troops. Gawain his killed during the shipwreck. His dying words to Arthur and not to trust Mordred. Arthur anxious to get to Camelot fights the Saxons not bothering trying to speak with them. Mordred prepares troops as he hears of Arthur fighting his way to Camelot. The two men meet to try to mediate their differences. Mordred comes to the talks with a white standard as he cannot have the King’s standard. The two are able to work out their differences, but as they make their truce a soldier is struck on the heel by a snake and draws his sword. This is the signal for battle so the two sides begin to fight. Mordred and Arthur attempt to stop it, but cannot be heard. They are swept into battle. Most of the men are killed, including Arthur and Mordred.