In the year 778, in a remote valley of the western Pyrenees, Eneko Ximenez, the son of the valley’s lord, seeks counsel from Luxa, a wise sorgina. Though he is nominally a Christian, his fear of a coming Frankish host under Charlemagne drives him to enter Mari’s dwelling to plead for the goddess’s aid.
When the rear of the Frankish column crosses a pass, Basques ambush them. In the heat of battle, Eneko performs a ritual that slits his own throat, and giant boulders rain from the sky, crushing the Franks and shifting the fate of the valley.
Meanwhile, Eneko’s son, also named Eneko, is separated from his mother and chased through the woods by a Frankish warrior. He hides by a river, and watches as a lamia bewitches the warrior and drowns him. He is found by a girl, Irati, who finds his fear of the woods amusing. Eneko watches as his father is buried according to the ancient ways, including the ritual execution of two prisoners from the battle. His grandfather, the lord Ximeno, arranges for him to be sent to Bigorra to be educated, and for his mother Oneka to remarry to Emir Fortun Ibn Qasi to secure an alliance with the Muslim Banu Qasi.
Fifteen years later, Eneko returns to his homeland. On the way to his grandfather’s fort, he encounters Irati, who has attacked a group of men sent to chop down the forest under orders from Belasko, a local chieftain. After making the men let her go, he arrives at the fort, where he reunites with his mother; his ailing grandfather rambles about a hidden treasure hoard, and asks to be buried with his son according to the Christian rite, before dying in Eneko’s arms. Eneko’s father’s body is found to have been replaced with that of a ram. The local chieftains convene in the castle, and have Luxa, who tried to stop the exhumation as blasphemy, imprisoned; Eneko, whose claim to the lordship of the valley has been contested by Belasko, visits her in the dungeon, where she reveals his father’s body is in Mari’s cave deep in the forest, together with the hoard taken from the Franks after the battle. Unbeknownst to them, they are overheard by a maid who promptly tells Belasko.
Eneko sets out into the woods, guided by Irati at Luxa’s behest; all the while followed by Belasko’s men. Before entering the cavern, Irati gives Eneko an eguzki-lore amulet for protection; he reluctantly wears it. Inside the cavern, Irati and Eneko are briefly attacked by Belasko’s men, and then by Tartalo, a man-eating, one-eyed giant; in the chaos, it is revealed Irati is not fully-human, having webbed duck-like feet like a lamia. Eneko falls down a pit.
Deep in the cave, Eneko finds [Mari] and makes a bargain with the goddess: she will return his father’s body, in exchange for the lives of Belasko and his cronies. Mari also reveals Irati is her daughter and tells Eneko to protect her. Emerging from the cave, Eneko tracks down the men and saves Irati. As they recover from wounds during the night, they fornicate.
With his father’s sword recovered from the hoard, Eneko returns to the fort, where he tries to lure Belasko into coming with him to the cave, assuring him there is no danger there. Belasko then reveals he knew the truth all along, and takes Eneko, his mother, and Irati prisoner. He then executes Luxa. Mad with grief, Irati’s eyes begin to glow as she summons a hailstorm. The soldiers and common folk run for shelter, and Eneko sees a ram take Irati upon its back, and fly her into the storm. She is taken to Mari, her mother, and tells her she will deliver Belasko and his men, which also means Mari won’t be able to take her along with the rest of her progeny when she makes her planned departure from a world where mortals barely remember her.
Belasko tries to torture the cave’s location out of Eneko, but Irati appears and leads the men to a cave. Mari appears before them, together with her serpent-consort Sugaar, who is revealed to be the creature that attacked them earlier. Irati slits Belasko’s throat, and Mari unleashes her power and departs from the world in the form of a fire blast, which kills all inside the cave except Eneko, who is shielded by Irati. When the smoke clears, Irati bids both her mother and him farewell, telling him to forget her name while he is lord. She then discards her clothes by a riverbank, and disappears in the water.
Eneko is crowned lord of the valley, and his realm grows into a kingdom; he names the forest where he lived his adventures as the Irati Forest. Many years later, now old, he travels with his heir to a riverbank, where he bequeaths his kingdom and floats down the river to die. He is found by Irati and he joyfully cries.
