He's merely leading his pursuers into a trap and they're soon surrounded by Arthur and the real knights - cue an impressive action-hero moment from the King as he draws his sword and leaps at the villains.
Agravaine identifies the leader of the invaders as Caerleon, ruler of a rival kingdom. Arthur is warned by his slippery uncle that, since Uther's death, many believe Camelot to be easy pickings. "To the enemies of Camelot, you are still untested as a King," he claims.
Uncle Aggy tells his nephew that he must make a stand - either Caerleon signs a treaty of complete surrender or he will be killed. But Arthur knows that their foe would rather die than give up his pride - will he be forced to kill in cold blood?
In the morning, a glum Arthur has decided to draw up the treaty regardless. Merlin warns his friend not to "humiliate" his enemy, but Arthur tells him to keep his nose out of his business. Arthur confronts Caerleon with his choice - sign the treaty or face death.
"Very well - then make it quick!" he growls. Arthur tries to reason with his nemesis, but Caerleon won't hear a word. "So be it," says Arthur grimly.
Back in Camelot, Arthur assures Merlin that the "show of strength" was necessary. "My conscience is clean," he insists, but Merlin tells the King that he is always around to help. Once again, his gesture of friendship is rejected - "The kingdom's my responsibility now - mine to bear alone," he's told. "You must learn to accept that."
But when Caerleon's body is returned to his own kingdom, his queen, Annis (guest star Lindsay Duncan) brands his death '"the work of cowards" and vows vengeance on Arthur and Camelot!
It seems that Agravaine's manipulation of Arthur has set a dangerous series of events into motion, but he's not done yet and has his sights set on Gwen. The slippery snake tells Arthur it is not "appropriate" for him to be dating a servant - "You must show strength again," he reasons. "Set aside those personal feelings for the good of the kingdom!"
Arthur doesn't take too kindly to this idea and Merlin is soon taking the worst of his frustrations as he assists the King with his physical training! But there's a larger concern on the horizon - the army of Queen Annis is advancing on Camelot...
The enemy troops will reach Camelot by sundown the following day, so Arthur decides to cut them off before they can breach the walls of the kingdom. He announces to his knights that they will ride out to face the aggressors at dawn.
That night, Morgana sneaks into Caerleon's castle, but she's captured by an eagle-eyed guard. Dragged before Queen Annis, Morgana tells the widowed royal that she sympathises with her plight and offer her assistance in destroying Camelot.
Meanwhile, a disguised Arthur has sneaked out of the castle and arrives at Gwen's home. But he acts cold towards his love, telling her that now he is King, it is "not appropriate" for them to be seen together. Harsh words!
Gwen is stunned, but as Arthur departs, she tearfully tells him, "You have a good heart - be true to it. Only then will you be the king you want to be."
In the morning, Arthur and the knights ride out on to the plains, while Annis and her army approach. As night falls over the land, Arthur declares, "The battle commences at first light!"
Once it's dark, Merlin has fun with the knights, drinking and larking around, but Arthur is reluctant to join in. And Merlin knows why - the responsibility of placing his friends and allies in danger is beginning to weigh heavily on the King!
Led by Elyan, the knights pay Arthur a visit and once again swear their allegiance. But once they're gone, Arthur tells Merlin, "I never once questioned their loyalty, [but] I do wonder if I deserve it."
Arthur now realises that he made the wrong decision in killing Caerleon. Guilt-ridden, he sneaks away from camp in the middle of the night, but Merlin stirs and follows. The King infiltrates the camp of Caerleon's army and tells the guard, "Take me to your Queen!"
Arthur is brought before Annis and gets a slap round the chops before he can get a word out. He tells the devastated queen that he is ashamed of his "cowardly" and "unjust" actions, then asks her a favour - to call off the battle between their armies in exchange for a single combat, thus avoiding further bloodshed.
If Arthur's man wins, then Annis must withdraw her army, but if her man is successful, then she may seize control of half of Camelot. Merlin has also been captured by Annis's goons, but the queen is merciful and spares his life. "Announce your champion by noon tomorrow," she tells Arthur.
The King returns to camp and informs his knights of the deal he has struck. Each man volunteers to be his champion, but Arthur knows that there can only be one choice. "This fight's mine!" he insists.
Annis is aware of Arthur's prowess in the battlefield and is concerned about the coming battle, but Morgana assures her ally, "Arthur will not win, because I have the power to ensure that he doesn't."
While Arthur sleeps, Agravaine steals his sword and places it into Morgana's hands. The weapon is engulfed by flames and enchanted with dark magic. "Arthur's as good as dead!" hisses Morgana.
As he prepares for battle, Arthur begins to realise the full extent of his recent mistakes, telling Merlin to pass on his apologies to Gwen if he does not survive. Finally the moment arrives - Arthur must face Darian, a gigantic brute of a man!
The fight begins, and though Darian towers over Arthur, the King is able to hold his foe at a standstill. But Morgana schemes to "turn the tide" and enchants Arthur's sword - it now holds the weight of a thousand ages!
Unable to lift his sword, Arthur cannot defend himself as Darian moves in for the killing blow. But with the help of Merlin's magic, Arthur is eventually able to wound his foe. Darian collapses to the ground, but Arthur has learnt his lesson and shows compassion, sparing his enemy's life.
Queen Annis tells Arthur that she will follow the terms of their agreement, but wants to know why he did not finish Darian off. "It is not victory I seek," explains the King. "It is peace." Annis seems satisfied by his reasoning and replies, "There is something about you, Arthur Pendragon, something that gives me hope for us all."
Morgana returns to Annis, but the queen tells her that she has made a mistake - she misjudged Arthur and was foolish to ally herself with a witch. "In my grief..." muses the queen. "...I let you infect me with your hate!"
Morgana tells Annis that she is weak and vows to destroy Camelot alone, but as she leaves, Annis can't resist getting in one last burn - "I fear you're more like Uther than you realise," she says.
Arthur and his knights return to Camelot, covered in victory. Gwen is summoned to the King's chambers and Arthur presents her with some flowers he picked from the road.
He apologises for his earlier foolish actions. and tells Gwen: "A good king should be true to himself." And first up on the new King's royal agenda - a snog with his true love!
• Another solid Merlin episode, 'His Father's Son' makes some bold choices - it's interesting to explore the ramifications of Arthur's poor decision-making and a brave move to have the character make mistakes, including killing a man in cold blood.
• But does anyone else feel that Gwen forgave Arthur a little too easily at the end? He really did say some cruel things - I'm not sure a scrappy bunch of flowers and a quick kiss cuts it!
• It's also a shame that the episode ends with yet another battle where Merlin secretly uses his magic to help Arthur - it's a trope that's getting more than a little tired after four years!
• We feel like it's time for Morgana to discover Merlin's powers - it'd certainly make for a compelling new twist and could provide plenty of material for storylines...
• We didn't see much of Gaius this week - hopefully he's plotting a way to topple Agravaine, who is still running riot and causing trouble, even though Gaius and Merlin fingered him as a baddie way back in episode two!