As Arthur welcomes a bellicose king, the Sarrum, from a neighboring realm, Merlin is surprised by a visitor, himself - a Druid boy who begs Merlin to cure his young sister of the sweating sickness. Merlin is compelled to help the young Druid and leaves Gaius to make an excuse for his absence, but as they travel further from Camelot, Merlin realizes a moment too late that the boy is not who he claims to be, and in that gap, Morgana knocks the wizard unconscious. She doses him with poison and heads for Camelot, where she instructs Gwen on their latest assassination plot.
The Sarrum's reputation for viciousness worries Arthur, but he needs the man's alliance if there is to be peace in the five kingdoms. At dinner, the Sarrum revels in telling Arthur of how he imprisoned and tortured Morgana, a tale which overwhelms Gwen with sadness and forces her to leave. But later, at the behest of Morgana, Gwen strikes an uneasy bargain with the warlike lord wherein he assassinates Arthur and she rewards him with one third of Camelot's lands. The brute agrees, and Gwen gives him a key that will allow his assassin to access an unguarded vantage point from which to shoot Arthur.
Half a kingdom away, Merlin revives enough to comprehend the agony he's in, but just then the boy who led him hence returns and insists on helping him. Merlin directs the young man to curative herbs, and manages to dispel the hurt, but there's still a long journey ahead of them, made urgent by the boy's information that Morgana and the queen plan to kill Arthur.
Merlin and his new ward arrive at Camelot just as the kings bend to sign the treaty, and the sorcerer notices a door that should be locked standing ajar. The assassin sees them and trains his bow on them, and Merlin deflects the arrow, which the assassin follows with two throws of quick daggers. Merlin levitates a spear at the man's heart and fells him, and a newly nocked arrow flies from its path towards Arthur and hits The Sarrum, instead. But the boy was also hit, and Merlin cradles him as he drifts toward death, relieved that his final deed was a good one.
The following day, Merlin's grief is deepened when he learns that Gwen explained his absence by telling Arthur that he was seeing a girl. That Gwen would lie in such a boldface way shows Merlin that her hatred for Camelot and Arthur continue to mold her into a more and more grotesque monster.