His Father’s Son opens with Merlin dressed as a knight of Camelot being chased by a small army of bandits. He deliberately luring them into a trap set up by King Arthur, where he waits with a small army of his own. After defeating the bandits, Agravaine hauls their leader to Arthur, revealing the man’s true identity. The man they captured is Caerleon, the ruler of a neighbouring kingdom who has been caught trespassing several times. Agravaine plays on Arthur’s insecurities as a new king, suggesting that their prisoner either agree to a treaty of what Agravaine thinks is fair or to kill him in the morning. Arthur does not sleep, the weight of this decision heavy on him.
Merlin tries to talk some sense into Arthur; the actions Agravaine wants him to take are clearly opposite of what Arthur would do. But since his uncle is his most trusted adviser and Merlin nothing more than a servant. Agravaine presents Caerleon with the treaty, and Arthur steps forward to take responsibility for the terms. When their captive declines to sign such a treaty, he tells Arthur to make it quick. Arthur tells Caerleon he should sign the treaty so there can be peace between their kingdoms, but Caerleon declines saying he is not his father, neither is Arthur his. Caerleon reasserts that it is his choice to die, not Arthur’s.
When they return to the castle, Agravaine praises Arthur for his actions, but when Merlin is alone with the king, he tries to make Arthur see he had other options. And when Merlin tells his king not to push away his friends, Arthur claims he has no choice. Right now he needs to show strength and doesn’t need friends. Later in the evening, Agravaine approaches Arthur and voices his concerns that Gwen would not make a proper queen in the eyes of his people and other kingdoms. Now that he is king, he must do what is expected of him and what makes his look strong. For the good of the people, Agravaine says Arthur must put aside his feelings for Gwen.
Caerleon’s men bring his body back to his wife, Queen Annis, where she declares Arthur will pay for killing her husband in such a cowardly fashion. After sending some of her army towards Camelot, Carlion’s wife receives a mid-night visit from Morgana. The witch comes bearing aid in the battle against Arthur.
Training is intense the next day as Arthur is still working through the events of the past few days. His regimen with the punching bag is interrupted when Sir Leon brings news of an army advancing on Camelot, lead by Annis. Arthur summons the counsel and the men begin to make their defensive plans. Merlin is worried that his friend has become too influenced by his uncle. As Merlin prepares Arthur’s quarters for the evening, Arthur leaves giving Merlin no idea clue as to his destination. He heads to Gwen’s, where he tells her that as king he can no longer be with her. Gwen believes he has been led to make this decision by others, but Arthur tells her he has come to this conclusion on his own. Before he leaves her home, Gwen reminds him that the only way he can be the king he wants to be is by being true to himself.
The next morning, Arthur leads his men as they ride out to meet with Annis’ forces. The men set up camp for the night on the ridge overlooking Annis’ legions, Arthur instructing his uncle to ensure the men have everything they need and get a good night’s rest; they will commence battle at first light. It’s obvious to his friends on the eve of battle that this fight is weighing heavily on Arthur. With Elyan leading them into his tent, he reminds Arthur that the men of his army are willing to give up their lives for him and for the freedom of Camelot. Arthur begins to doubt if he is worth such affection and loyalty, and his choices leading him to this point.
When his men have all fallen asleep, Arthur sneaks out of the camp, Merlin following close behind. Merlin watches from the rocks as Arthur approaches two guards at Annis’ camp and asks to see her. Merlin peeks into the tent and witnesses Arthur confess his cowardly actions to the grieving widow, and his invocation of the right of single combat. Arthur would be his own representative and Queen Annis can choose her champion. The winner of the duel will be the victor of the battle, thus sparing the men from fighting. If Arthur wins, Annis’ men will withdraw; if Arthur loses, he will give her half of his kingdom. Not a moment later, Merlin is discovered and brought into the tent. Luckily, Merlin’s appearance does not ruin Arthur’s attempts and Annis agrees to the duel to take place at noon.
Before the duel, Arthur informs his trusted circle of the terms he arranged with Annis the previous evening. It is clear on Agravaine’s face that he is none too happy about this turn of events. All of his knights offer to fight as the king’s champion but Arthur is determined to fight himself. When Annis finds out, she believes it to be some sort of trick. Morgana confirms that Arthur is the type of man who would indeed spare the lives of his men, and reminds the queen that she has the power to make sure Arthur fails. Agravaine steals Arthur’s blade, bringing it to Morgana so she can cast an enchantment on it. Before he heads to the duel, Arthur has asked Merlin that should he fall in battle to give his ring to Gwen and give her his apologies for how he acted.
With his men watching from the cliff and Annis’ men on the field, Arthur and the queen’s Champion walk to the center of the empty grassland and duel begins. A few minutes into the battle, Morgana activates the enchantment which causes Arthur’s arm to become heavy as it increases the weight of his blade. With Arthur on the ground and about to be skewered, Merlin casts his own spell, forcing the Champion to drop his sword. This gives Arthur the opportunity to take him from behind and knock the large warrior to the ground. He could easily kill the queen’s man, but he chooses to spare him, doing the honourable thing.
Annis approaches Arthur, telling him she will adhere to the terms of their agreement and asking why he spared the life of her champion. Arthur tells her that it is not victory that he seeks, but peace. Annis believes that there is hope for the people of the Five Kingdoms in Arthur, and leaves to bring her people home. Morgana enters her tent, claiming things will be different next time. Annis informs the witch there will be no next time for her, as she is not consumed by hatred as Morgana is. Morgana sees Annis’ actions as weakness, blind to the fact that her lust of vengeance is corrupting her. As she leaves the tent, Annis tells Morgana she is much more like Uther than the girl realizes.
The episode ends with Gwen entering Arthur’s chambers in the morning. He gives her some wildflowers that he picked and apologizes for his behaviour. Gwen accepts his apology and they make up with a kiss.