In The Crystal Cave, readers follow the story of the wizard Merlin from the time he is a young boy until the time just before his legendary protege, King Arthur, is born. This classic fantasy novel provides an in-depth exploration of Merlin's formative years, detailing his early mastery of his magic, which manifests itself in clairvoyant visions, and his alliance with High King Ambrosius. The first in a quintet of books exploring these ancient legends, The Crystal Cave begins Mary Stewart's imaginative retelling of the King Arthur legends.
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 plays to their superstition and pretends to attribute the problems to dragons beneath the ground. (The dragon is Ambrosius's emblem.) As a result of this, Merlin briefly 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 fulfilled when a comet appears in the sky and Ambrosius dies. Uther, Ambrosius's younger brother, becomes King Uther Pendragon. However, Britain is thrown into chaos when Uther, besotted with Duchess Ygraine, goes to war with her husband, Gorlois, the Duke of Cornwall. Merlin helps Uther to enter Tintagel Castle by stealth, knowing the tryst will lead to the birth of King Arthur.