Set in "Britain's land beyond the seas" during the Age of Chivalry, The Lay of Aotrou and Itroun tells of a childless Breton lord and lady (the "Aotrou" and "Itroun" of the title) and the tragedy that befalls them when Aotrou seeks to remedy their situation with the aid of a magic potion obtained from a corrigan, or malevolent fairy. When the potion succeeds and Itroun bears twins, the corrigan returns seeking her fee, and Aotrou is forced to choose between betraying his marriage and losing his life.

Originally written in 1930 and long out of print, this early but seminal work was the outcome of a comparatively short but intense period in Tolkien's life when he was deeply engaged with Celtic, and particularly Breton, myth and legend. Like The Legend of Sigurd and Gudrún,The Fall of Arthur, and The Story of Kullervo, it belongs to a small but important corpus of Tolkien's ventures into "real-world" mythologies, each of which in its own way would be a formative influence on his own legendarium.

Edited with notes and commentary by Verlyn Flieger and a prefatory note on the text by Christopher Tolkien.