Bayard and the Knights of the Western Isles are welcomed to Camelot for a feast celebrating a union between the two kingdoms. However, Nimueh has other plans. Posing as a serving girl, she infiltrates Bayard's party and enters Camelot, seeking revenge on Merlin.
Unaware of her true identity, Merlin befriends the young serving girl. Once she has gained his trust, Nimueh warns him that Bayard isn't the friend he seems - he plans to poison Arthur. Merlin bursts into the banquet and stops Arthur drinking from a poisonous goblet in the nick of time. Bayard denies that he tried to poison Arthur and Uther decides that the only way to determine if he did is for Merlin to drink from the cup himself. Merlin falls to the ground, poisoned. The guards arrest Bayard and his Knights.
Gaius discovers that Merlin has been poisoned by the Morteaus Flower and can only be saved from certain death by an antidote made from the leaf of the very same plant. However, the flower is rare and only grows in the caves of Mount Argeaus, which are found deep within the forest of Balor. As Merlin's health deteriorates fast, Arthur, humbled by his servant's bravery, believes he must set off on a quest to retrieve the flower himself. However, Uther forbids him to go because the danger is too great. Morgana counsels Arthur and helps him realise that he is his own man and should do what he thinks is right, even if it means disobeying Uther.
Sneaking out in the dead of night, Arthur sets off on his quest. Braving the perilous forest of Balor and a vicious Cockatrice, he manages to reach the cave where the flower grows. What he doesn't know is that he's walking in to a trap. The witch, Nimueh, is waiting for Arthur. She leads the young Prince into the caves, claiming to know the location of the flower. Once inside, Nimueh begins to mumble a spell, causing the rocky outcrop on which Arthur stands to crumble...Arthur narrowly avoids falling and clings perilously to a small ledge. Nimueh turns and bids him farewell, taking the only light source with her as a swarm of giant spiders creep towards him. But help comes from an unexpected quarter and Arthur is able to escape with the antidote in hand.
Arthur races against time to get back to Camelot before the poison kills Merlin. He arrives at the gates but, just when it seems like all will be well, Arthur is arrested for disobeying his father. Merlin is at death's door and it falls on Gwen to sneak into the dungeon and get the flower from the incarcerated Arthur. Gwen gets the flower to Gaius and Merlin is saved. However, celebrations are short lived as Gaius gives Merlin some sobering news: Nimueh was after Merlin all along. She must know his importance to Camelot and is determined to stop him at all costs.
Morgana and Mordred prepare to destroy Camelot (and Morgana plots to destroy Emrys). Is Merlin Camelot€™s last hope?
Merlin star Colin Morgan talks about the important events from episode 511 'The Drawing of the Dark.'
Mordred battles between his duty to his friends and that to his love, but is spared the choice.