The dark sorceress Nimueh casts a spell on a dragon egg which she throws into the castle's water supply. The next day, people start dying of a plague which turns their skin white with dark blue veins. Uther orders Arthur to search all Camelot for the sorcerer he believes to have caused the epidemic - only a magic trick saves Merlin from having his book found. When Gwen's father exhibits symptoms, Merlin disregards Gaius's warning by curing him. However, when Arthur reports the recovery, Uther believes Gwen to be the sorceress and sentences her to death. With some prompting from Morgana, Arthur goes with Merlin to hunt down the monster (an afanc).