Merthin is one of the main characters of the live action series "World Without End" and along with Caris (who he loves) is one of the main heroes in the show, being one of the nicer, friendlier, and more benevolent characters.
Merthin first appears in the series premiere engaged in a practice duel with his brother Ralph, who repeatedly bests Merthin with ease. Their duel is interrupted by the arrival of the knight Sir Thomas, who after killing his pursuers collapses from his injuries. Merthin and Ralph then take him to the Priery where he can heal.
Merthin and Ralph's father is later arrested and hanged by Sir Roland for treason, forcing the boys to then seek out someone to apprentice themselves to. Roland forces Merthin and Ralph to fight each other, saying that he will take on the first brother to draw blood. Merthin loses the fight, and is apprenticed to Elfric, who wastes no time abusing him. Merthin goes on to become a good craftsman in his own right, even taking on some independent projects on the side. Ever-abusive and petty, Elfric never passes up an opportunity to belittle and verbally abuse Merthin, also abusing him physically when he criticizes Elfric's own building ability, which Merthin regards as sloppy.
Merthin has strong feelings for Caris, which do not disappear after her marriage to his master. When Caris attempts to get Merthin work, Elfric learns of this and hits her in his rage, also vowing to do everything in his power to keep Merthin out of the guild. Disgusted by his master's actions, Merthin attacks him and beats him, even attempting to kill him before Caris stops him. Merthin warns Elfric that he will finish the job should he ever strike Caris again. Elfric then orders Merthin and Caris out of his house, and following the collapse of the town's bridge, Merthin offers to help Caris build a place for the sick and injured to reside. Caris decides to consider his offer, also admitting to returning Merthin's affections but not yet being ready to make a marital commitment.
After Caris is confined to a nunnery to escape execution for witchcraft, a heartbroken Merthin leaves Kingsbridge. Though he settles down with another woman and with her starts a family, they die during the plague, and a disheartened Merthin eventually returns to Kingsbridge, at last reuniting with Caris. Merthin retains his feelings for her despite her being forced to be a nun, and the two eventually confess their feelings for one another in full. Feeling that they do not have a future in Kingsbridge so long as Godwyn rules with an iron fist, the two plan to leave, but Sir Thomas produces a document that would free Kingsbridge of the Priery's control, also revealing himself to be King Edward II in disguise at the same time.
When King Edward III comes to Kingsbridge with his army to raze it, Merthin helps lead the defense, and battles his evil brother Ralph. Though Merthin loses to Ralph, he is saved from death by Gwenda and Wulfric's son, who shoots an arrow into Ralph's head and kills him. After the battle, Merthin and Caris embrace in each other's arms, at last able to be together free of Godwyn's influence.