A man is seen enchanting a beetle which then crawls inside a flower he is holding. A bunch of these flowers is anonymously left for Morgana. The man, whose face is heavily scarred, watches her window from the ground below. As she sleeps that night, the beetle comes out of the flower and goes into her ear.
Two days later, Morgana has become dangerously ill with an inflammation of the brain. Gaius cannot find the cause and does not know what to do for her. Merlin offers to try and help her using magic but Gaius reminds him that his previous use of a magical cure nearly led to Gwen being executed. The scarred man arrives at the castle and introduces himself to Prince Arthur. He is Edwin Muirden, a physician who claims to have "a remedy to cure all ills." Arthur turns him away but Edwin promises to be at the inn should he be needed.
Morgana is now only hours away from death. Arthur becomes so afraid for her that he tells his father about the mysterious Edwin. King Uther orders Edwin to be summoned at once. Gaius believes he and Edwin have met before but Edwin denies this. He asks for privacy while examining Morgana; Gwen enters the room unawares but he angrily confronts her and orders her to bring water. When she leaves, he uses a spell to remove the beetle.
Edwin tells the king that Morgana has in fact had a cerebral haemorrhage and indirectly accuses Gaius of incompetence in treating her. Uther is delighted at her miraculous recovery. When questioned by Gaius, Edwin again says they have never met. A suspicious Gaius asks the court genealogist, Geoffrey of Monmouth, for records of the "Great Purge" when Uther removed magic from the kingdom. However, Geoffrey is too afraid to open the records because they both know the king would forbid it.
While looking at Edwin's magical equipment, Merlin finds a box containing what appears to be dead beetles. There is a magical enchantment carved into the lid. He is able to read it and when he does so, they come to life. Edwin catches him and is impressed by his magical ability. He encourages Merlin not to be afraid to use magic to do good, and offers to be his mentor. Merlin says that Gaius will not allow him to use magic but Edwin insists magic is a wonderful gift, and by working together they could become great.
Geoffrey of Monmouth trusts Gaius enough that he produces the old records after all. While at dinner with Uther, Edwin casts doubt upon Gaius's capability as a physician. Later that evening Gaius confronts Edwin with his new found knowledge: Edwin's parents were old friends of Gaius, a witch and wizard executed on Uther's orders when he removed sorcery from Camelot. Edwin reveals that he knows about Merlin, and that his face is scarred because he tried to save his parents from being burnt at the stake. He believes they died because of Gaius, who was also a sorcerer but chose to save himself rather than his old friends. He threatens to kill Uther if Gaius tells anyone, including Merlin.
Edwin now tells Uther that Gaius has become too incompetent in his old age to continue as court physician. Gaius visits the Great Dragon, who states it is Merlin's destiny to unite Albion alongside Arthur, and "their time cannot come until (Uther's) has passed." He says it is now Gaius's choice as to whether that will happen sooner rather than later. Gaius refuses to sacrifice Uther for Merlin so the Dragon tells him to turn a blind eye as he did before.
Uther dismisses Gaius from the job of court physician, thanking him for his many years of loyal service. Gaius resolves to leave Camelot. He tells Merlin that they have been like father and son, and reminds him of his great destiny. The next day Edwin poisons Uther, replacing the potion that Gaius had given him to treat an old wound. Gwen encounters Gaius as he is leaving. She does not trust Edwin, but Gaius says there is nothing he can do. Edwin has paralysed Uther and inserts one of the magical beetles into the king's ear. It will eat into his brain and kill him within hours.
Gaius returns to the castle, swearing that Edwin will not kill Uther. He could not bring himself to help Edwin's parents because he knew they were using evil magic. Edwin encases Gaius in a ring of magical fire but Merlin walks in and Gaius tells him the truth. Merlin refuses Edwin's offer to rule the kingdom together using magic. Because of this, Edwin enchants an axe from the wall to fly towards Merlin. Merlin magically returns the axe towards Edwin and Edwin is killed. Gaius and Merlin remember the king and rush to his side. Merlin is reluctant to use magic to save Uther but Gaius tells him it is the only way. Using his own magic rather than Edwin's, Merlin removes the beetle and Uther is cured.
A recovered Uther believes it is Gaius who saved his life. He reinstates Gaius to his job and makes him a free man of Camelot, stating (in private) that Gaius is the only man he can trust in the fight against magic.