This novel covers the time from Merlin's sixth year until he becomes a young man. The Romans have recently left Britain, which is now divided into a number of kingdoms loosely united under a High King. Merlin is the illegitimate son of a Welsh princess, who refuses to name his father. Small for his age and often abused or neglected, Merlin occasionally has clairvoyant visions. These visions and his unknown parentage cause him to be referred to as "the son of a devil" and "bastard child". Educated by a hermit, Galapas, who teaches him to use his psychic powers as well as his earthly gifts, Merlin eventually finds his way to the court of Ambrosius Aurelianus in Brittany. There, he assists in Ambrosius's preparations to invade and unify Britain, defeat Vortigern, the Saxon usurper, and become its High King. Also exiled in Brittany is Uther, Ambrosius's brother, heir and supporter.
It is revealed that Merlin is Ambrosius's son, the result of a brief relationship between Ambrosius and Merlin's mother. Merlin returns to Britain but finds Galapas killed. He is captured by Vortigern who is attempting to build a fortress at Dinas Emrys - but each night the newly built walls collapse. The king's mystics say the fort will only be built when a child with no father is sacrificed and his blood spilt on the ground. Vortigern plans to use Merlin as the sacrifice. Merlin realises that the fort's foundation is unstable due to the caves below ground, but he attributes the problems to dragons beneath the ground. As a result of this Merlin becomes known as Vortigern's prophet. Days later Ambrosius invades and defeats Vortigern.
Merlin uses his engineering skills to rebuild Stonehenge but has visions of Ambrosius's death which are soon fulfilled when a comet appears in the sky and Ambrosius dies. Uther becomes King Uther Pendragon. However, Britain is soon thrown into chaos when Uther, besotted with Duchess Ygraine, goes to war with her husband the Duke of Cornwall. Merlin reluctantly helps Uther enter Tintagel Castle by stealth, knowing it will lead to Arthur's birth.