Merlin Episode 5.07 A lesson in Vengeance

When Arthur's horse throws him into the path of murdering bandits, it's discovered that his saddle was cut and resewn, an offense that only could've been perpetrated by the stable boy, Tier. When Arthur questions him, he denies guilt and says that no one has been in the stables but him. Arthur cares for the boy and doesn't want to harm him, but he must be found guilty of treason and sentenced to death. But when Merlin talks to him, Tier gives a halting admission that someone cut the saddled and threatened to kill Tier's mother if he told. Merlin runs to tell Arthur, but the king is persuaded by Gwen not to see the boy until morning. Merlin and Gwen eye each other suspiciously, but neither can speak their revelations.

Gwen later steals to the castle jail and stabs Tier to keep him from revealing her as the assassin. Arthur frets over the knowledge that someone in his inner circle made an attempt on his life while Gwen meets outside the grounds with Morgana, who instructs her to find another innocent to take the fall for Arthur's assassination, but they're interrupted by a patrol, and Gwen tears her dress on a branch. Soon, Morgana delivers powerful tinctures to Gwen, who poisons Arthur at their supper that night. When Arthur is found unconscious, Gwen declares Merlin the only person capable of slipping the poison to Arthur, and the knights rally around Gwen as the successor to the throne. She hides her triumph behind tears of modesty and throws Merlin in jail.

But Gaius visits Merlin and tells him he must find a way to save Arthur with his magic. Gaius delivers a familiar bottle to Merlin, one that transforms him to an old man, and after sneaking through the grounds, Merlin scales the wall into Arthur's chamber. With every drop of strength he has, he pours his magic into Arthur to dispel the poison, and at last, Arthur's pulse steadies and his color returns. Merlin returns to his cell, where he is pardoned the following morning by his revived king.

Arthur later finds and interrogates the apothecary who delivered the poison, and the man reveals that the tincture was purchased by Lady Morgana. Arthur praiseshis queen for her shrewd assessment of the scandal, but Merlin sees clearly that he has a strong foe in Morgana's new ally, won over when she was imprisoned in the Dark Tower with dark magic.