A sorcerer opens a box of beetles and takes one out. He casts a spell on it, then places it in a flower bouquet, where it hides. He sends the bouquet anonymously to Morgana, which Gwen (Guinevere) brings to her chambers. At night time, as Morgana sleeps, the bouquet of flowers by her bedside, the beetle emerges from the flower and crawls into her ear. The man, whose face is heavily scarred, watches her window from the ground below.
Two days later, Morgana is dangerously ill with a mystery illness. Uther demands answers from Gaius, who's been treating her, but so far all his remedies have failed. He tells Uther that he thinks Morgana has an inflammation of the brain, possibly caused by an infection. He assures Uther he'll do everything in his power to find a cure. Outside her chambers, he tells Merlin she's as good as dead. When Merlin offers to help her using magic, Gaius reminds him that his previous use of a magic to cure Gwen's father nearly led to her being executed for witchcraft.
The scarred man arrives at the castle and asks to see Prince Arthur. When they meet, he introduces himself as Edwin Muirden, and that he has a remedy to cure all ills. Arthur dismissively tells him that they already have a court physician and walks off. When Edwin says that he's heard that Morgana is ill, Arthur tells him that it's no concern of his and that their physician is treating her. Edwin tells Arthur that he'll be staying at the inn should he change his mind.
Morgana's condition has worsened and Gaius' diagnosis is that she's now only hours away from death. Arthur tells Uther and Gaius about his encounter with Edwin and his claim to cure all ills. Both Uther and Gaius are skeptical of his claim, but Arthur sees no harm in trying anything they can to save her. Uther summons Edwin to the castle, and when Gaius sees him, he thinks they've met before. Edwin denies this, referring to his scarred face, and saying he doubts Gaius would forget one like it. He shows reverence to the legendary Gaius, praising his treatments, then asks if he can examine Morgana.
Arthur orders Merlin to assist Edwin. In Edwin's guest chambers, Merlin admires his impressive apothecary set. He picks up the closed box of beetles, which Edwin takes from him, saying they'll need it. In Morgana's chambers, Edwin asks for privacy while examining her. This includes Gaius, who intended to stay. As Edwin treats Morgana, the box of beetles nearby, Gwen enters the room and watches. He senses her presence and asks why she's spying on him. When she explains she's Morgana's servant, he orders her to fetch water. After she leaves, he uses magic to remove the beetle, then blots the bleeding caused by its egress from her ear.
Edwin emerges from Morgana's chambers with good news that she's improved. He says that it wasn't an inflammation of the brain as Gaius had diagnosed, but a cerebral hemorrhage, indirectly accusing Gaius of incompetence in treating her. He shows them the bloody handkerchief, which he says came from her ear. When Uther asks Gaius how he could have missed this, Gaius says with conviction that he didn't see any blood. When Uther sees Morgana conscious and talking, he's overjoyed at her miraculous recovery. Gaius asks her what Edwin gave to her, but she has no idea. She's just glad that he did.
Later, Gaius congratulates Edwin on bringing Morgana back from the brink of death. He asks him how he did it. Edwin says that he's developed an elixir for the treatment of brain hemorrhages. When Gaius asks for the ingredients, Edwin says it's not perfected, and he needs more time before making it public. Gaius asks him about the injury to his face, which Edwin says happened when he was a child. When Gaius suggests that he may have treated him for it, Edwin insists that they've never met.
Uther asks Edwin to name his reward. Edwin refuses payment, saying he's only interested in the well being of his patients. He does ask to remain until Morgana is fully recovered and then he'll be on his way. Uther offers to let him stay at the castle. Edwin graciously refuses his "kind offer," stating he feels he'd be trespassing. Plus, the king already has a court physician. Uther says Gaius would be grateful for his help and tells him to consider it. He invites him to dine with him later and give him his decision then. Edwin tells Uther that to dine with him is reward enough.
Suspicious of Edwin, Gaius asks Geoffrey of Monmouth to open the records from "The Great Purge." Though they are old friends, Geoffrey says this is the one thing that he cannot do for Gaius. Even though Gaius says that he asks because it's of great urgency, Geoffrey still won't do it, as the records are hidden, sealed and Uther has forbidden it.
While snooping though Edwin's apothecary set, Merlin opens the strange box and sees the dead beetles. He shuts the lid and reads aloud the enchantment etched into it. Upon reopening it, he sees they've come to life. Edwin catches him and is impressed by his magical ability. At first, Merlin denies using magic, but Edwin tells him not to be afraid. He stills the beetles with an enchantment, saying they saved Morgana's life. When Merlin says Gaius doesn't like him to use magic, Edwin says he should use and practice his gift, stressing the good it can do. He offers to teach him, saying, "Imagine what we could achieve if we shared our knowledge."
After a change of heart, Geoffrey visits Gaius to give his trusted old friend the Great Purge records. He says he knows Gaius wouldn't ask if the stakes weren't high. Grateful, Gaius assures him they are. Meanwhile, Edwin dines with Uther and casts doubt upon Gaius' capability as a physician. He says the sleeping potions that Gaius was giving to Morgana to quell her nightmares, masked the symptoms of the underlying problem. At that, Uther asks if he'll undertake a review of Gaius's work just to be on the safe side. Feigning humility, Edwin agrees to do it.
His suspicions confirmed, Gaius confronts Edwin, saying he wondered what happened to that little boy he treated for burns so long ago. At first, Edwin insists they've never met, but Gaius says he knew his parents, who were sorcerers that Uther executed when he banned magic from Camelot. Cornered, Edwin threatens to spill Merlin's secret. Edwin, who was burned trying to save his parents from the stake, asks who will try to save Merlin. Gaius says he wouldn't sacrifice Uther to save Merlin, but Edwin warns him to think about that, and to keep this quiet or he'll tell Uther.
With the stakes raised, Edwin tells Uther that after reviewing the records, he "regrets" to report that Gaius's methods are outdated and have led to errors. When Uther says Gaius has served him well for 25 years, Edwin suggests that Gaius has become incompetent in his old age. Uther ponders this, then agrees it may be time to lighten Gaius' load. He asks Edwin if he's reconsidered his offer, to which Edwin responds that he has. Uther says he'll ponder it as well. Morgana defends Gaius to Uther, but he's firm, not wanting to risk a mistake like that happening again.
Gaius visits the Great Dragon, whom he hasn't seen for 20 years. He's surprised to hear that the Dragon already knows about Merlin. The Dragon confirms that it's Merlin's destiny to unite Albion alongside Arthur, but their time cannot come until Uther's has passed. Gaius wants to know if that's now, and the Dragon cryptically answers that it's Gaius' choice as to whether that will happen sooner rather than later. When Gaius refuses to choose between Uther and Merlin, the Dragon tells him to turn a blind eye and quips, "That is after all, your talent."
Uther summons Gaius to tell him that he's retiring him as court physician, a difficult decision since he considers him a friend. He thanks him for years of loyal service and says he'll give him a generous allowance and living arrangements. Gaius asks if it's because of the mistake, which he still contends he didn't make. And, if Edwin will be replacing him. When Uther answers yes to both, Gaius is on the cusp of telling Uther the truth, until Edwin enters the room and practically dares Gaius to speak up. Gaius stays silent and bids Uther goodbye.
When Merlin learns of Gaius' fate, he wants to do something. However, Gaius is resigned and says he's leaving Camelot. In protest, Merlin wants to leave with him. Touched, Gaius tells Merlin he considers him a son. Merlin likewise considers him a father. So, speaking as a father, Gaius reminds Merlin of his destiny, which requires him to remain in Camelot. Determined to save Gaius' job, Merlin talks to Arthur, who says that Edwin said Gaius' work was riddled with errors. When Merlin calls that nonsense, Arthur says no one wants Gaius to go, but Uther has made his decision.
As Gaius is leaving, he encounters Gwen, who says she doesn't trust Edwin and knows he made Morgana sick. She pleads with Gaius not to go, but he says he has no choice. That night, Edwin switches the potion that Gaius prescribed for Uther's old battle wound with a paralyzing potion. Edwin wakes the immobilized Uther to tell him who he is and that he's here to take revenge. He places one of the magical beetles near Uther, who's helpless to stop it from crawling in his ear. It will eat into his brain and kill him within hours.
Gaius returns to Camelot and tells Edwin he won't allow him to kill Uther. Edwin says he never minded letting people die before. Gaius counters that his parents practiced dark magic, "just like you." He tries to cast a spell on Edwin, but gets it wrong. Edwin laughs that Gaius is out of practice and says it correctly, slamming him against a wall. He forges a ring of fire around Gaius when Merlin walks in. Gaius tells him of Edwin's plans to kill Uther. Edwin tries to entice Merlin to join him in ruling Camelot with magic. When Merlin refuses, Edwin commands an axe to hurl towards him. However, Merlin enchants it back toward Edwin, and kills him.
Before Merlin entered Edwin's chambers, Arthur told him that Uther had Morgana's illness. He tells Gaius and takes the box of beetles, which Gaius recognizes as Alanthia beetles -- they can be enchanted to enter the brain and eat it until they devour the soul. At Uther's bedside, Merlin is at first reluctant to use magic, but Gaius says it's the only way, and Merlin's right to do so. Merlin fears he doesn't know how, but faced with Uther's death, he improvises a spell that successfully removes the beetle. Amazed, Gaius says, "Has anyone ever told you you're a genius?" Merlin wittily responds, "You certainly haven't."
Gaius tends to a recovering Uther, who asks him if he remembers Edwin's parents. Gaius remembers them all. Uther apologizes for betraying him and swears to remember that in the fight against magic, Gaius is the only one he can trust. With full ceremony, Uther not only reinstates Gaius as court physician, but honors him by naming him a Freeman of Camelot. Privately, Gaius tells Merlin it's ridiculous, as he saved Uther, not him. Merlin is happy that Gaius, who risked his life for Uther, got credit. When Gaius realizes that Merlin saved both him and Uther, he jokingly tells him that maybe he's a genius after all. Almost.