The Lost Years of Merlin is an adventure tale based on the idea that as a young boy, the legendary wizard Merlin spent several years in a mystical realm. Drawn there by his desire to uncover his forgotten past, he discovers the land is suffering from a curse and blight. The tree-girl Rhia convinces him to aid her in lifting it. They embark on a nearly impossible quest to face the malevolent Rhita Gawr and his puppet, King Stangmar, in their concealed fortress. Along the way, they encounter various beings—benevolent, malevolent, deceptive, and enigmatic—who either assist or obstruct their mission. Rhia is captured and taken separately to the castle, while Merlin, alongside a tiny giant and riding the hawk Trouble, manages to overcome the final barrier, a haunting dark landscape, and enters the castle.

Inside, Merlin and Trouble temporarily defeat Rhita Gawr, and young Merlin confronts the king. Stangmar reveals he allied with the evil being to save his wife's life, which required sacrificing his son by age seven. However, the mother and child escaped, leading to the blight on his land. It is no surprise by now that young Merlin is the son. In a conflicted struggle of conscience, Stangmar alternately rejoices at finding the boy and attempts to kill him. Merlin must battle him to free the land from its curse. Fincayra's hidden giants awaken and bring down the Shrouded Castle in a dramatic conclusion.

This story resonates with numerous mythic themes. The father-son dynamic recalls tales as ancient as Abraham and Isaac in the Old Testament, and as modern as Luke and Anakin Skywalker in the Star Wars saga. The quest journey borrows elements from the Welsh story cycle, the Mabinogion. The first third of the book is set in "real life" fifth or sixth-century Wales, a region recently influenced by the Romans and not yet besieged by Saxons or Viking invaders. The tale concludes with the boy adopting the name "Merlin" in honor of his hawk, setting the stage for further adventures.

In his journey, young Merlin, referred to as Emrys for much of the story, gains considerable knowledge and begins to develop a sense of responsibility towards others. However, he still has a long way to go before he becomes the legendary Merlin. He is just beginning to tap into his extraordinary magical abilities and lacks worldly wisdom. In folklore, Merlin is credited with orchestrating the birth of Arthur through a spell that makes Igraine see King Uther Pendragon as her husband, Gorlois, Duke of Cornwall. The young Merlin in this series neither possesses the magical prowess for such feats nor the cunning required to execute them. Illustrating the wizard's journey to develop these attributes would likely require another series, centering on his life in early middle age—a scope beyond that of these books.

The Lost Years of Merlin—along with its sequels—offers a captivating exploration of a vividly imagined fantasy realm, filled with thrilling adventures and notable character development.

The book also delves into moral complexities. How steadfastly should one adhere to a promise made in a moment of despair? What if the promise is made on your behalf? How much should children know about the unfortunate circumstances surrounding their birth? Is it ethical to involve others in a perilous quest, even if they can assist you? In a world filled with illusions, how can one discern what is real? These nuanced questions, combined with young Merlin's connection to the natural world and its wisdom, transform the book into a fantasy quest that is richer than the sum of its parts.