Bent on having her revenge on Merlin after he killed the Afanc she set on Camelot, Nimueh creates a poisoned chalice. In Camelot disguised as a servant, she watches as Lord Bayard, the king of Mercia, arrives in the castle to solidify his truce with King Uther. Afterward, Nimueh passes by Gaius and Merlin in the hallway and purposely trips. Merlin helps her right herself, and she bats her baby blues at him, introducing herself as Kara. Merlin is clearly smitten with her. Nimueh sneaks into Bayard's quarters and switches one of the ceremonial goblets with her poisoned creation.
At the feast, Bayard speaks about their newly formed alliance as Merlin, dressed in Camelot's ostentatious ceremonial servant's garb, sneaks peeks at Nimueh. Gwen (Guinevere) notices and admits she's pretty for a handmaiden. Merlin says she's pretty even for a princess. Bayard presents Uther with a gift of two goblets; one for the king and one for Arthur. Nimueh takes Merlin aside and tricks him into believing that she saw Bayard lacing Arthur's goblet with poison. She begs him not to say that it was she who told him.
Just as they are toasting, Merlin snatches the goblet from Arthur and says that Bayard poisoned the goblet. Bayard draws his sword at the insult, but is outnumbered by Camelot's guards. Uther asks who witnessed this, but Merlin won't tell. Uther tells Bayard that if he's telling the truth, he has nothing to fear. Bayard offers to drink from the goblet, but Uther wants Merlin to do it. If it's poisoned, Uther tells Bayard he wants the pleasure of killing him himself. Though everyone tries, Uther won't be dissuaded on making Merlin drink from the goblet. If he lives, Merlin will be given to Bayard to use as he wishes.
As Nimueh watches, pleased that her ruse has worked, Merlin drinks from the goblet and appears to be fine. However, seconds later he collapses. The Camelot guards take Bayard into custody as Arthur and Gaius carry Merlin back home. Gwen brings the chalice, in which Gaius finds a poisonous mortaeus flower petal. As Gwen tries to keep Merlin's fever down, Gaius tells Arthur that an antidote can only be made from the leaf of the very same flower, found in caves deep beneath the Forest of Balor. Even worse, a fearsome creature, the highly venomous Cockatrice, guards the forest. Few who crossed the mountains of Iskard looking for the flower have made it back alive.
Nimueh, her work finished, leaves the castle by horseback. Gaius tells Arthur that death from the mortaeus is slow and painful, and that Merlin has four to five days at most. Arthur, feeling responsible for Merlin's plight, wants to retrieve the flower, but Uther forbids it, as he's the only heir to the throne. Despite Arthur's protests, he's not about to let his son die for a servant. However, Morgana later convinces Arthur that his father isn't always right. Arthur bursts through the castle guards and races out into the night. From her mirror, Nimueh watches the action.
While Gaius and Gwen care for Merlin, he mutters in a foreign language that Gaius recognizes as a magical tongue. When Gwen asks what he's saying, Gaius blames it on his fever. Gaius notices a rash on Merlin that shouldn't be there this early in the poisoning and realizes that someone enchanted the poison to increase its potency. This means that Bayard, who isn't a sorcerer, couldn't be responsible. Gaius suspects Nimueh and remembers the "servant girl" who called Merlin aside at the ceremony. He sends Gwen to find her in the dungeons where Bayard and his staff are imprisoned, but she isn't there.
To Morgana, Uther expresses his fury at Arthur for disobeying him. When she doesn't act surprised, he realizes that she knew about Arthur's plans. She tells him that he must let Arthur make his own decisions, but Uther counters, "Even if it means his own death?" At that, Morgana doesn't have an answer. Gwen tells Gaius that she didn't find the servant girl. Gaius realizes that since Nimueh knows where the only antidote can be found, Arthur may be walking into a trap.
On his way to the caves, Arthur comes across Nimueh, who's disguised herself as a maiden. Weeping, she claims that her master beat her. Suddenly, a cockatrice attacks. As Arthur fights it, Nimueh can barely disguise her glee at the duel. However, Arthur turns his sword into a spear and skewers the beast through the heart. When Arthur tells the "maiden" he's looking for the mortaeus flower, she says she knows where to find it and offers to show him. Meanwhile, in between speaking the magical tongue, Merlin mumbles about Arthur going into a trap.
In the cave, Nimueh points to the flower on the other side of a rock bridge. When Arthur crosses, she casts a spell to make the bridge collapse under his feet. Arthur jumps and catches himself on the other side, clinging to a small ledge. Stunned, Arthur demands to know who she is. She tells him she's the last face he'll ever see. A giant spider attacks Arthur, and he kills it with a swipe of his sword while dangling by one hand. Nimueh says she'll let its friends finish him off, as it's not his destiny to die at her hands.
In a trance-like state under the effects of the poison, Merlin mumbles about Arthur's peril. As an astonished Gaius watches, Merlin creates a sphere of light in his palm, which appears in a larger form by Arthur. At first, Arthur thinks the sorceress is taunting him, but then realizes it's showing him a way out. As Arthur struggles to reach the flowers just feet away, an army of giant spiders advances from below. Despite the oncoming danger, he continues his detour and grabs the flower. Aided by Merlin's light and just feet ahead of the hoard of angry arachnids, Arthur scales the cliff to an exit from the cave.
Upon his return to Camelot, Arthur is arrested by order of the king, and thrown into the dungeon. He begs Uther to take the flower to Gaius, but Uther, more interested in teaching Arthur a lesson than saving a servant's life, crumples the flower and drops it outside the cell. He callously tells him that he can find himself another servant. After his father leaves, Arthur reaches through the bars to try to get it back. Meanwhile, Merlin's condition worsens.
Gwen tells Gaius that Uther won't allow anyone to see Arthur, so she doesn't know if he got the flower. To find out, Gwen gets through the guards by bringing a plate of food to Arthur, saying it's his dinner. When Arthur gets his plate, he feigns being disgusted at the offering and tells Gwen to take it away. When she does, she sees he's placed the flower on the plate. Aside from almost being found out when another servant brings food, she gets back to Gaius, who gets to work on an antidote.
In order to make the antidote work against the enchanted poison, Gaius realizes he must use magic, which is forbidden. He sends Gwen on an errand and uses magic to strengthen the antidote. With Gwen's help, he pours the antidote down Merlin's throat. At first, it appears Merlin has died. Gwen and Gaius weep in each other's arms. That is, until Merlin teases Gaius for smooching a girl young enough to be his granddaughter. Both are ecstatic he's alive, Gwen so much that she kisses Merlin. He asks them what happened, as he has no memory after drinking from the goblet.
Gaius interrupts Uther as he's planning for war against Bayard's army to tell him that Nimueh is responsible for poisoning the chalice. At first, Uther doesn't believe him, but when Gaius says that magic was used to bolster the poison, he tells his men to hold off on leaving Camelot. After assuring Uther that he's made the right decision, Gaius asks him if he thinks that Arthur should be told the truth about Nimueh.
When Morgana asks him how he managed it, Arthur said he had help. Someone knew he was in trouble and sent a light to guide the way. Uther asks Arthur if the woman in the forest told him anything. Arthur says that she could have finished him off, but didn't, and said it wasn't his destiny to die at her hands. Uther reiterates that those who practice magic are evil, which is why she wanted Arthur dead. He tells Arthur that he did the right thing even though he disobeyed him. And, to never forget that he's proud of him.
Arthur visits Merlin, who thanks him for saving his life. Arthur chides that half-decent servants are hard to replace. However, all kidding aside, Merlin sincerely thanks him. And, Arthur thanks him as well. After Arthur leaves, Gaius praises Arthur's noble act, saying not many would risk their lives for a servant. Merlin says it would all be for nothing if Gaius didn't know how to make the antidote. When Merlin asks why she didn't kill Arthur, Gaius says that destroying Camelot wasn't her only goal. She knows Merlin is destined for great things. Nimueh watches through her basin, worry etched on her face.