After Arthur is attacked after falling from his horse, stable boy Tyr Seward (John Bradley) is accused of sabotage, but Merlin suspects the boy is innocent.
Visiting Tyr in jail, Merlin learns that the lad’s mother will be killed if he reveals the identity of the person who messed with the king’s horse. Gwen – who is now Morgana’s puppet following her ordeal in the Dark Tower – then silences Tyr before Arthur can question him.
Meeting in secret to discuss their next move, Morgana and Gwen are almost caught by Sir Gwain, but the knight is knocked off his horse. Gauis and Merlin then set out to find evidence that Morgana is behind the plot – whereupon Merlin discovers a scrap of expensive silk.
Meanwhile, having secured two vials of poison from an alchemist, Morgana slips the vials to Gwen who then laces Arthur’s drink. Suspecting Gwen, Merlin riffles through her wardrobe and finds a match. But he is too late to stop Arthur drinking the poison. Gwen then accuses the young sorcerer of the deed, whereupon he is thrown into jail.
Realising only Merlin’s magic can save Arthur; Gaius gives Merlin an aging spell so he can slip out of the prison. When the alarm is raised that an intruder is at large, Gwen is taken to her room for protection. This gives Merlin the chance to work his magic on Arthur and then return to jail unnoticed.
The episode ends with Merlin and Gaius keeping silent about Gwen’s involvement, while the alchemist is made the fall guy in Morgana’s plot.