Deep in a subterranean cave a beautiful witch, Nimueh, molds a clay model into a tiny creature that she brings to life with a drop of water. She places the strange being into an egg and sends it hurtling down the river that flows towards Camelot.  The egg surfaces in the water of a darkened vault – the one which supplies all of Camelot’s drinking water.

There’s a terrible sickness spreading through Camelot. Uther demands to know the cause. Gaius is forced to admit that it is a magical plague. Uther orders Arthur to find the culprit.

Merlin tells Gaius that if the plague is a magical illness he wants to try to cure it with magic but Gaius turns on him – is he mad?! If he’s caught practicing magic, he will be killed. They need to use science to find the source of the disease so they can prevent it spreading.