Just a quick recap of the Return of Nimueh:
At the end of Out of Time, Nimueh trapped Alethea as she was sent home by Merlin. Alethea was then tied up and held prisoner whilst Nimueh took on the guise of Alethea and returns to Camelot for her revenge, leaving the College student trapped and alone. Meanwhile, Prince Arthur is distraught after Alethea left without a word and him being too late to stop her. He fights in the tournament that day and nearly loses his head as he sees Alethea in the crowd. After winning, he searches for her but doesn't find her until the next day where they meet in the marketplace. Nimueh's guise is so good that Arthur suspects nothing and so the two of them go back to the castle. Merlin is not informed of this, and does not want to see the Prince as he is guilty for sending Alethea back. The warlock instead spends most of his time reading the book that the real Alethea had written. The fake Alethea at that time is busy keeping up the charade of being in love with the Prince whilst secretly raising an army to attack Camelot.
Meanwhile, the real Alethea suffers alone, being tied and gagged, not knowing what is going on and praying that Nimueh would not kill the Prince. As she does not know what Nimueh plans to do with her, she finds herself steadily weakening and wonders if she will die here.
At Camelot, Merlin discovers that the returned Alethea is in fact Nimueh but he is knocked out before he can tell the Prince. When he does recover, Arthur does not believe Merlin until it is too late. Nimueh manages to raise an army and plans to attack Camelot at first light, but the town is warned just an hour beforehand. Quite a few people fall in battle and Nimueh is the victor, with Uther being her prisoner and Merlin and Gwen under her influence. The Prince got his wish of going to Alethea, but it cost him his life and so the whole of Camelot mourns for the loss of their Prince.
Alethea, still bound, sees Arthur appear and her heart leaps with joy to see him come to rescue her, but he collapses. Managing to free herself, she goes over to him and finds him dead. Sitting by her Prince, she mourns him and goes over just what had happened to them since they had first met. Knowing that there is no way back, Alethea accepts her fate whilst Camelot suffers. However, Arthur recovers and scares Alethea half to death before they announce their love for each other. Happy together, they sit and talk until they fall asleep in each other's arms.
That evening, Nimueh arrives and gives Alethea a proposition. She has to kill Arthur and she'll be sent home, otherwise they both die. Alethea reluctantly accepts but plans to trick the sorceress. However, it all gets too much and Alethea plans to leave the Prince whilst he is still sleeping. The Prince wakes and drinks from the flask that Nimueh had given to Alethea. To late to stop him drinking it, Alethea carries the dead Arthur to Camelot. Here Uther sees Alethea and his dead son and vows vengeance. Alethea waits for the final and fatal blow but it never comes. Instead, she manages to free Merlin from Nimueh's influence before the four of them escape into the forest where Alethea is blamed for Arthur's death. The Prince decides at that point to reveal that he was only sleeping before Alethea collapses from exhaustion. Arthur returns to Camelot to gather what he needs to save her but Nimueh blocks off his route. The sorceress gives the Prince the chance to save Alethea's life if he will give himself up as prisoner. Arthur considers Nimueh's offer but turns it down before he runs into the survivors of the battle. Arranging to meet with them the following day, he returns to Alethea where she recovers later that evening. They plan how they are going to free Camelot and defeat Nimueh before putting it into action.
Merlin and Alethea take on Nimueh whilst Arthur, Uther and their men free the prisoners in the dungeons. Getting them to safety, Arthur then goes to join Merlin and Alethea where he finds Alethea suffering from Nimueh's spells. Arthur and Merlin then fall victim to Nimueh's spells whilst Alethea is left to face her alone. Knowing that she is about to die, Alethea stares at Nimueh and accepts what is about to happen. Only by chance does Alethea manage to get rid of the sorceress by reflecting one of the spells with a shield before the student blacks out.
When she recovers, she finds that Camelot is back to normal with Arthur and Merlin both recovered. Preparations are being made for a feast but no one will tell Alethea what exactly is going on. Given a dress by Arthur, Alethea gets ready for the feast and wonders just what exactly is about to happen. Meanwhile Arthur and Merlin talk, and Merlin reveals that he is a magician to Arthur. The Prince accepts this and makes Merlin promise to send Alethea home should the need arise, before they go to the feast. Here Alethea is made a Lady whilst the three of them are made Protectors of Camelot. Alethea then dines with the King the next day whilst Arthur goes out riding. The Prince then takes Alethea out riding the following day where he proposes to her. Alethea turns him down before running away and sobbing. She eventually returns to Camelot, regretting her decision and, after supporting Arthur at the soldiers' funeral, she tells him that she has changed her mind. The two of them are married soon after.
The Prince and Princess of Camelot with their two servants then go on a journey around the Kingdom but half way through they are set upon by a band of cutthroats. Alethea is held hostage and is about to die with Arthur powerless to save her when Lancelot appears and saves them. Alethea is now worried about the arrival of Lancelot as she finds herself slightly attracted to him but knows the legends and rides off. Arthur chases after her where Alethea explains what she is thinking and promises to him that she will not fall in love with Lancelot. She then persuades her husband to knight him but then finds herself ignored as Arthur and Merlin are desperate to hear just what Lancelot has been up to since he's been away and Ceridwen is always near him. Feeling neglected, Alethea separates herself from the group and retires early on the last night of their journey, knowing that tomorrow they will return to Camelot and see just what, if anything, has happened whilst they have been away.