On a hunting trip, Arthur kills a unicorn. Merlin rushes to the animal's side, obviously upset, as the Knights cheer Arthur for killing this most highly-prized of beasts. As the unicorn dies, Merlin sees a wizened old man watching the scene amongst the trees. Their eyes meet, and then he's gone.
The next morning, Arthur and Uther survey a valley. The crops in the fields below have died overnight. It's the same story all over the kingdom. When, later that day, the water in Camelot's well turns into sand, Arthur and Uther become desperate - the town is now facing a famine and a drought. Gaius suggests that these events cannot be a coincidence - a curse has come to Camelot.
That night, Merlin and Arthur are on patrol when they come across a strange man; it's the same person Merlin saw in the forest. He introduces himself as Anhora, the keeper of the unicorns. Anhora tells them Arthur is responsible for the curse, revealing it was unleashed when Arthur killed the unicorn. Anhora tells Arthur that he will be tested, and until he has made amends for his actions, Camelot will be damned.
The following day, Arthur is still refusing to accept that he is responsible for the curse. He thinks Anhora is to blame and has a plan to catch him. He and Merlin will stake out Camelot's grain store together, Arthur suspects this is where Anhora will strike next. Arthur and Merlin see someone sneaking into the grain store. They give chase and corner Evan. Evan explains he is a desperate father who is only stealing food because his children are hungry. Uther has ordered that looters be executed, but Arthur takes pity on Evan and lets him go. Arthur even lets Evan take some food for his family.
The following morning, the sand in the well has turned back to water. Merlin suggests that it's because Arthur proved himself worthy by showing mercy to Evan. Arthur realises there may be something in what Merlin's saying. With the food reserves still running low, Arthur realises he must do what he can to lift the curse.
As Arthur searches for Anhora in the forest, he finds Evan's camp. From the goods lying around, it is obvious that Evan is a thief. Arthur reacts with contempt as he realises Evan lied to him about stealing food for his children. Evan starts to mock Arthur, laughing at how he fooled the great Prince. Arthur lifts his arm, preparing to run Evan through with his sword, but as he does, Evan vanishes. Arthur now sees Anhora looking on from the edge of the clearing. Anhora says Arthur has killed a man in anger. Arthur chose to protect his pride when he should've turned the other cheek. Arthur has failed the test, for which Camelot will pay the price. Anhora vanishes. Arthur fears the consequences of what he has done.
Merlin returns to the forest to seek Anhora out. Merlin pleads that Arthur deserves another chance. Merlin knows that Arthur can prove himself. Anhora says Arthur must face the ultimate test at the Labyrinth of Gedref.
Merlin tells Arthur of the test and offers to accompany him, but Arthur will not risk Merlin's life. It is Arthur that brought this curse upon Camelot and it will be he that lifts it or dies trying, but Merlin cannot leave Arthur to face the test alone. Merlin secretly follows Arthur when he leaves Camelot.
Arthur eventually finds the center of the maze but it's not a huge monster waiting for him - it's Merlin, sitting at a table. Anhora appears. He places two glasses between them and tells them that one of the glasses is poisoned, one is not. Both glasses must be drunk. They must choose which one they will each take. They can only drink from one glass each. If Arthur passes the test, the curse on Camelot will be lifted.
Before Merlin can stop him, Arthur pours the contents of both glasses into one and downs them without pause. He falls to the ground and seemingly dies. Merlin is devastated, believing he has failed in his destiny to protect Arthur, but Anhora reveals that Arthur isn't dead. He has merely consumed a sleeping draft. As Arthur was prepared to give his own life to save Merlin, he has passed the test and proved himself worthy. The curse will be lifted from Camelot. Anhora disappears into the maze as Arthur starts to come around.
Arthur and Merlin return to Camelot to find the grain in the stores is once again edible, the wheat in the fields has re-grown and the well is full of water. Arthur makes Merlin promise not to tell anyone that he was willing to die for a servant, and he promises not to tell anyone that he had to save Merlin's life - again.
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.