Merlin Episode 5.11 The Drawing of the Dark

On a hunting expedition, Arthur and the knights come across a wagon that was full of weapons bound for Camelot, but it's been sacked by Saxons. Mordred spies something running. When the others catch up to him, he says he was mistaken, but Merlin sees him let a Saxon girl go. When he steals into the woods with medicine and food, we learn that is his childhood love, Kara, and her leg is badly wounded. Merlin notices his absence and confronts Mordred, but promises not to reveal the secret on account of the girl being a druid.

Later, when Arthur hunts with Merlin, the two discover tracks that lead them straight to Kara, who attempts to murder Arthur. Merlin foils her and they return to Camelot to try Kara, which sends Mordred into a rage, believing Merlin betrayed him. In her trial, Kara shows no remorse for her act, nor makes any attempt to soften her sentence, and thus she is condemned to hang. Mordred is deeply in love with Kara and reveals himself as the one who took her medicine, begging Arthur to repeal the sentence. Arthur cannot set free someone so dangerous, however, and sadly denies the plea. Merlin sees something turning inside Mordred, recognizes the danger, and adds his voice to Mordred's, but to no avail.

Later, Merlin sees Mordred slipping down the hall to rescue Kara. He reasons with Mordred that this will do no good, but the knight won't hear, leaving Merlin with no choice but to warn Arthur. When the pair is caught, Merlin once again pleads with Arthur to free Kara, and this counsel weighs on him so heavily that he offers the girl one more chance to repent her crimes in return for forgiveness of them. But Kara believes so firmly in her cause that she spits his mercy back in his face and walks proudly to the gallows. As Mordred sits in his cell, he feels the death of the woman he loves like a thorn in his heart, causing him to release a cry so powerful that the iron cage he's in splits open and he escapes into the world.

Arthur realizes he's made a terrible mistake, and Mordred plods his way to Morgana's stronghold, where he reveals the true identity of Emrys.