In the previous episode, Merlin and Arthur stopped Nimueh's Afanc from doing more harm. Nimueh was outraged when she realized that Merlin had used magic to help Arthur defeat the Afanc, and so she soon created a poisoned chalice to kill Merlin.
Nimueh disguises herself as Kara, a servant of Bayard, a king that has been at war with Uther for a while. There is going to be a celebratory banquet. Nimueh, as Kara, gets Merlin to trust her by pretending to trip. Merlin is stunned by her beauty.
Once the chalice is ready, Nimueh switches it with the original one.
At the banquet, Merlin is on duty, standing in the attire of a servant, when Nimueh, showing distress, comes and brings Merlin into a hallway, telling him that the chalice is poisoned. Her reasons for this will be shown in the next part.
Merlin rushes into the banquet hall, just as the men were about to drink, and says that Arthur will die if he drinks out of the chalice. Bayard is accused, because the chalices were his. He denies that he poisoned it. Uther does not want to take a chance, and orders that Merlin drink it to make sure. Merlin does, and at first he seems to be fine. He drops to the ground, choking. Arthur, Gaius, and Gwen rush to Merlin, and Bayard can only stare, after taking out his sword. The three bring Merlin to Gaius's chambers. Merlin has a fever, and Gaius proclaims that the Mortaeus Flower was what created the potion, and that the Mortaeus Flower is what can create the antidote. Then we see Nimueh leaving the castle by horse. Arthur is trying to convince his father to let him go for the flower, but Uther refuses, saying that he will not risk the life of the only heir to the throne. Later, Morgana is talking with Arthur, who is angry that Uther will not let him go, but knows that there would be dire consequences if he, (Arthur) died. Morgana asks, "And what kind of king would Camelot want? One who would risk his life for that of a lowly servant, or one who does what his father tells him to?" And hands him his sword. And then we see Arthur on his horse, leaving the kingdom.
Gaius and Gwen are tending to Merlin, who's fever has gotten worse. While doing this, Gaius notices a rash on Merlin's arm. Rushing back to his book, Gaius tells Gwen that the rash occurs only in the final stage. He realizes that someone has increased the potency, something that can only be done by magic. He says that Merlin has two days...and then, a thought occurs to him. He tells Gwen to find the servant girl that Merlin was with...'Kara'. Gwen goes down to the dungeons, where Bayard and his people that he brought with him, are being kept. She looks at them all, but does not see Kara/Nimueh. Merlin, delirious, starts to mutter incantactions.
Uther and Morgana are talking. Uther is furious that Arthur left, and while conversing with Morgana, realizes she knew. Morgana defends Arthur, saying that he is old enough to make his own desicions.
Arthur has reached the woods, and has found a clearing. There, a young lady in tattered clothes, with a scratch on her arm, is weeping. This is Nimueh, a fact that Arthur is oblivious too. While they talk, an animal has been seen. The animal attacks Arthur, and while he fights it, Nimueh smiles evilly. But Arthur defeats it, and Nimueh asks him to help her. He tells her that he has business to finish, but he won't leave her. She asks him if it is the Mortaeus Flower he's after. She says that she knows where it is, and starts leading him into a cave. Merlin mutters, back at Camelot, that it's a trap.
They reach a place similar to the area where Merlin meets the Dragon. It's a small cliff edge overlooking a chasm. At a small break between the rock they are standing on, she points to a small ledge, that is a jumping distance from the farthest point of the cliff part that they stand on. Arthur starts to walk to the edge closest to the ledge with the flowers. Nimueh starts to chant under her breath, the chant growing stronger and louder. Arthur feels the rock crumbling, he glances at her, and jumps toward the ledge. His torch fell into the chasm, he is hanging by his hands. As he hangs, a huge, probably poisonous, spider crawls along the edge. Arthur manages to kill it with his sword, (still hanging). Nimueh just laughs, saying that the rest'll finish him off. She leaves, leaving him in darkness. Merlin, feverish, murmurs, (unconciousely) "Arthur...too dark..." He starts chanting, and Gaius, watching him sees a bright light grow from under the blanket. He whips back the blanket, to see an orb in Merlin's right hand, glowing with blue light.
Back in the cavern, still hanging, Arthur sees an orb, emitting a blue glow, rise out of the darkness. He thinks it's there to kill him, saying, "Go on, then. What are you waiting for? Finish me off!"
The orb merely floats a foot above him, illuminating the ledge. Arthur hoists himself up, collapsing on the ledge. The orb rises higher, and with its light, Arthur sees the flowers, sitting in a cleft a little ways above him.
Merlin, back at Camelot, mutters, "Leave them, Arthur," Arthur hears the scuttling of the other spiders crawling up the stone wall of the chasm. Merlin is moaning, "Go, Arthur, save yourself. Follow the blue light," Arthur is climbing up the rock, and reaches for the flowers, managing to pluck one, which he puts in a pouch hanging by his side. The spiders are still getting steadily higher, and as Arthur climbs, the orb guides him, going higher. Merlin cries out, "Faster, go faster, follow the light!" Arthur keeps climbing, heading for the opening up above. He keeps looking back, and Merlin urges him on from his bed at Camelot, "Move, climb!" Arthur follows the light out, and once outside, runs off to get his horse. Back in Camelot, Merlin calms, the blue orb disappearing from his hand.
Arthur has made it back to to Camelot, and at the gates, a group of guards block him. "What are you doing? Let me pass!"
Arthur has been put into a cell in the dungeon. Uther was the one who put him there, and is angrily asking Arthur for the reason he disobeyed Uther's wishes. Uther believes that Arthur's life is much more important then a servant boy's. Arthur gives him the flower, and says that he can be locked up for a month for all he cares, just get the flower to Merlin. Uther crushes the flower in his hand, while Arther can only stare in disbelief. Uther tells him that he needs to learn respect, and leaves. Arthur reaches for a few petals that have fallen to the ground.
Gaius has Gwen go to the dungeons, carrying food for Arthur, although she wasn't supposed to. She enters the cell, and gives Arthur the food. Arthur sends her back, saying that the food was horrible, but Gwen sees the flower hidden in the middle of the plate, when she takes it.
Back with Gaius, he starts to create the antidote. Halfway, he stops, realizing that magic might be needed to create the antidote, because magic was used to create it. Gwen says that even if they could, magic was forbidden, so they were going to have to do their best. But Gaius sends her for fresh water, and while she is gone, he mutters an incantation over the antidote. When Gwen comes back, he finishes, and they give him the antidote. Merlin stops breathing, his heart has stopped.
Gwen is crying on Gaius's shoulder, when Merlin awakens, healthy and alert. He asks what happened.
Uther is planning attack plans with his men, because the imprisonment of Bayard has reached Bayard's land, Mercia, and they have sent men to attack. Gaius comes in, taking Uther aside. He says that Bayard is innocent, Nimueh was the one. Uther orders his men to change plans.
Arthur and Morgana are talking, Arthur recounting how the light helped him. He doesn't know who, but he suspects someone sent the light to help. Uther comes over and asks about the woman Arthur met.
Later, Arthur visits Merlin, checking to see if he was alright. He claims it was to see if he was fit to go back to work...but then, they both thank each other. Merlin, for the flower, Arthur for the drink.
When he leaves, Gaius tells Merlin that someone is out to kill him. Nimueh.