Ursula K. Le Guin, whose series of Earthsea books have delighted children, adolescents, and adults alike for three decades, has one of the most refined moral sensibilities of any fantasist. True, she presents herself as a political eccentric (a nonviolent Taoist anarchist, if I understand her theology correctly). True, outside of her fiction, she is a loud and public bearer of (often simplistic) grudges against much of systematized Western society. But the long, complex, and closely observed tale of power and obligation she spins in the Earthsea books far outstrips the schoolboy luddism of J.R.R. Tolkien or C.S. Lewis’s soppy-stern millenarian Christianity.

“Lavinia” (Harcourt, 288 pages, $24) is a recasting of the second half of Virgil’s “Aeneid” through the eyes of the Latin princess whom Aeneas is destined to marry, and with whom he will father the kings of Rome.

There is no work of epic literature more estranged from the spirit of the epic than the “Aeneid.” And the nature of this estrangement — that its hero is not a master of physical violence or tactics, but rather a sufferer for sociopolitical and spiritual ideals — suggests there was more than a little of Aeneas in Ms. Le Guin’s Ged, the disfigured hero of the Earthsea books, who begins as a precocious and arrogant boy wizard and finishes as a man willing to sacrifice his magical gifts to preserve his world spiritually and politically. He becomes, in other words, a creature fundamentally attuned to bonds physical and metaphysical. If anyone was to adapt any portion of the Aeneid, Ged offers strong evidence that Ms. Le Guin would be an ideal candidate.

Her story is simple: Lavinia is the independent-minded but dutiful daughter of Latinus, king of Latium, a principality on the Mediterranean coast of Italy. Numerous local princes have offered themselves as contenders for her hand, but Turnus, of the Rutulians, has found favor with her mother. Latinus realizes, however, that she must marry the just-arrived Aeneas in order to fulfill a prophecy. (Lavinia has been made aware of the prophecy, along with her own literary status, through a visitation from Virgil himself, plucked out of his wanderings with Dante.)

Aeneas and Turnus go to war and Aeneas dispatches his rival. There Virgil’s work ends; Ms. Le Guin takes the story on a bit further, charting the establishment of the city of Alba Longa by Aeneas’s son Ascanius and the birth and maturation of her own son with Aeneas, Silvius (who will go on to father the Julian line of kings). “Lavinia” ends with its heroine as an old woman, ruminating on her strange role in history and literature and on the decay of worldly power.

There is, conceptually, much to be said for Ms. Le Guin’s approach. The rearrangement of literary works to establish new tensions and illuminate new internal landscapes is an old and enormously fruitful practice. Virgil himself approached the primordial material of Homer with a renovator’s eye, and any number of modern and contemporary writers and other artists — from the German novelist Hermann Broch to the British poet Christopher Logue — have reshaped mythical, classical material into works that, however problematic, at least show a certain autonomy and vigor. But, for all her sympathy for Virgil’s vision, Ms. Le Guin fails to meet that standard.

It’s true that Virgil’s work looms over hers and makes it seem trivial, but this would be true of any writer — let alone someone without any formal or informal training as a reader of Latin, particularly as associatively rich and subversive an author as Virgil. The titanic stature of the author she has chosen to engage does not excuse the paltriness of “Lavinia,” however. So why has Ms. Le Guin failed?

In large part because her prose, which can most charitably be described as workmanlike, comprises a severe limitation on the book:

There were many who saw Turnus die, for it was before the gates of Laurentum that he finally stopped hiding from Aeneas and turned to fight him. Both men threw their spears, and missed. So they met sword to sword, but Turnus’s sword broke and he turned and ran again. Aeneas tried to chase him but was too lame to run. He stopped and tried to pull out his spear from the wild olive tree it had hit. That was a sacred tree. I had done worship to Faunus there many times.

“Lavinia” also lacks the mercilessly-plotted, enterprise-redeeming intrigue of Robert Graves’s “I, Claudius” — a similarly shallow work by a writer far more erudite, and one that, I suspect, has provided intense and guilty pleasure to aspiring students of classical antiquity for several generations. Or of Ms. Le Guin’s other books: She has always shown more strength as a constructor of gripping, affecting plots than as an artist in prose. But here she has managed, in her over-attention to Lavinia’s vague interiority, to siphon out all of the propulsive energy inherent in the last movements of the Aeneid, and leave us with a stilled, becalmed transcript.

It seems clear that Ms. Le Guin harbored no epic ambitions for her book. Which may, indeed, have been her central problem: To contend with titans requires arrogance and outsize will. But Ms. Le Guin chose to examine the rich and troubled inner life of a wise-beyond-her-years girl confronted with her romantic and sexual destiny. Even in this small fragment of an epic, there’s enormous potential — a spurned prince, a war, a forced marriage that fosters real love, an insane mother. But Ms. Le Guin makes almost nothing of this rich material, instead allowing over-ample time to Lavinia’s flaccid introspections (and this is from the book’s concluding paragraphs):

My hearing is good. I can hear a mouse breathe among the fallen oak leaves … I can hear the endless sound of engines of war on all the roads of the world. But I stay here. I fly among trees on soft wings that make no sound. Sometimes I call out, but not in a human voice. My cry is soft and quavering: i, i, I cry: Go on, go.

Only sometimes my soul wakes as a woman again, and then when I listen I can hear silence, and in the silence his voice.”

“Lavinia” is, in the end, proof at once of Ms. Le Guin’s narrow limits as a writer and of her considerable gifts — evinced in this case by their yawning absence. Robert Graves’s book has, in its small way, partaken in Horace’s dictum on his own literary work: “Exegi monumentum aere perennius, I have erected a monument more lasting than bronze.” So have Ms. Le Guin’s tales of Earthsea, which will live on in the hearts and minds of lonely, imaginative children and the adults they become. “Lavinia,” sadly, will not much outlast its season.

Mr. Munson is the online editor of Commentary.