Deep in an underground cave, beautiful witch Nimueh brings a clay model of a tiny creature to life with a drop of water. Nimueh puts the creature in an egg and sends it down the river leading to Camelot, and it ends up in a vault that supplies all of Camelot's drinking water. As sickness spreads throughout Camelot, King Uther demands to know the cause. Gauis admits that it's a magical plague and Uther orders Arthur to find the culprit. Merlin tells Gaius that if the plague is a magical illness, he wants to try and cure it using magic - but Gaius reminds Merlin that if he's caught practicing magic, he'll be killed. Meanwhile, Gwen's father falls ill with the disease and she goes to Gaius for help. When Gaius offers no hope for a cure, Merlin decides he must help and, ignoring Gaius's advice, secretly cures Gwen's father with his powers. But he's shocked when, the next morning, Gwen is arrested for using magic. When Uther concludes that Gwen caused the sickness, then used magic to cure her own father, he sentences her to death. Can Merlin save Gwen without exposing his own abilities?
