The battle at Camlann rages mightily, with Morgana clearing paths for her forces by using her powerful sorcery and Aithusa's fire. Merlin, in the disguise of his aging spell, makes his way to the battlefield at lightening speed. He commands Aithusa to leave the battle, then casts his own force onto the field, knocking over dozens of soldiers with a flick of his staff. Before Morgana can retaliate, he sends a blast of energy to her that knocks her unconscious.
The battle is over. The Saxons make their retreat. Gwen sends knights to look for Arthur, and when no sign is seen, she keeps herself busy by caring for the many wounded. Merlin also searches for Arthur, but Mordred reaches him first and uses the moment of Arthur's bewilderment to strike a fatal blow. Arthur falls to one knee and Mordred feels a moment of relief before Arthur summons his strength and pushes his sword through Mordred's stomach. Arthur withdraws his sword and stumbles once again to the earth.
After much searching, Merlin finds Arthur and spirits him away to a quiet glen, and when Arthur awakes, Merlin unburdens himself of his greatest secret. Arthur at first disbelieves, but when Merlin displays a small trick of magic, Arthur orders Merlin to be gone. When Gaius arrives, summoned by the young warlock, he tells Merlin that Arthur can't be saved by physic: there's a piece of sword, crafted in a dragon's breath, lodged in Arthur's core. Only the Sidhe, who dwell at the lake of Avalon, have the power to draw it out. Arthur wants no business with Merlin, but Gaius tries to assure Arthur not only of Merlin's goodness, but also his greatness as a sorcerer, and so the two friends begin the trek toward the lake.
Gaius returns to Camelot and, at Arthur's request, gives the royal seal to Guinevere. She's comforted that Arthur is alive yet grieving that he's so wounded, and mystified that he should make such a trek with only Merlin as guard. Gaius redirects her focus by alerting her to the betrayal of Gwaine's companion, Eira. They set a trap for the girl by telling her that Arthur and Merlin are headed in the direction opposite to their goal. The following day, Eira is executed, and Gwaine and Perceval set a course for Morgana.
When they come upon her, she is far too powerful for them. She knocks them out and ties them up, then presents a mystical creature to Gwaine that pulls Arthur's location out of him. Perceval manages to free himself from his bonds just in time to comfort Gwaine as he dies. Morgana flies toward Arthur as the king begs Merlin to rest for a while. Though he still doesn't trust Merlin, he begins to see that Merlin has used his deceit to do much for Arthur and Camelot, and begins to soften his attitude.
They resume their journey, but Morgana finds them by daybreak, thinking she has won, but not counting on the dragon-forged blade that Merlin has in hand. He drives it through her belly, finally killing the high priestess and bringing peace to the land. The horses fled at Morgana's approach, and Merlin drags his friend as far as he can until the king orders Merlin to stop. With ragged breath, Arthur thanks Merlin for all he has done in silent service. Arthur slips into unconsciousness, and Merlin calls for the dragon to fly them the rest of the way to Avalon. The beast sets them down and tells Merlin that his quest is over, but he has not failed. Arthur's time has long been foretold to end early, and the king and his sorcerer have done much to bring Albion into a better age. Arthur is not gone, not completely, for he is the Once and Future King, and when he is needed most, he will return. Merlin sets Arthur on a boat made of rushes and pushes him into the lake, then tosses Excalibur after him. The sword is caught by a strong hand.
With heartache and celebration, Camelot crowns Gwen as their ruler, and she moves forward in Arthur's vision. And far away, in our present time, a lone old man with a long, white beard walks the roads of England. He pauses to think at the lake of Avalon, a light in his eye - then walks on.