The Story of Merlin depicts the role of the seer Merlin in the conception and birth of Arthur, who is to rescue Britain from the Saxons and establish an ideal kingdom. It follows Arthur's career as he is designated king by the magical sign of the Sword in the Stone, triumphs over his rebellious barons and drives out the Saxons. With his marriage to Guinevere, he acquires the Round Table, and sets up the famous order of knighthood which is at the centre of his power.
The Merlin was written after the last three romances of the Vulgate Cycle were already complete, and serves as a prologue to the history of Arthur, just as the History of the Holy Grail is the prologue to the adventures of the Grail.
For a full description of the Vulgate Cycle see the blurb for the complete set.