Helena first appears in the Historia Regum Britanniae of Geoffrey of Monmouth, written around 1136, as the niece of Duke Hoel of Brittany, the cousin and ally of King Arthur who had fought beside him against the Saxons and accompanied him on the campaign against the Roman Emperor Lucius. Hoel was one of Arthur's most trusted commanders, a man who had led battalions at the decisive battle of Saussy and who was left in charge of Gaul when Arthur returned to Britain to face Mordred's rebellion. The relationship between Arthur and Hoel was close, forged in war and blood, and when word reached the army that Hoel's niece had been carried off by a giant, Arthur did not hesitate. He abandoned the preparations for the Roman campaign and rode to Brittany with his two most trusted companions, Cei the seneschal and Bedwyr the butler, to rescue the maiden and punish the monster who had dared to touch a woman of the Breton court.

The giant who kidnapped Helena was a monster of "marvellous bigness" who had come from Spain to make his lair on the summit of Mont Saint Michel, the rocky island that rises from the sea off the coast of Normandy. He was a creature of immense strength and cunning, who had already defeated every knight who had tried to reach him. When ships approached the mount, he hurled mighty boulders from the summit, sinking them before they could land. When knights managed to swim to the island, he killed them before they could gain a proper footing on the treacherous rocks. He was a man-eater, a brute who devoured his victims as was his wont with all those whom he killed, and Helena was his latest prize. The giant had seen her, desired her, and carried her off to his fastness on the rocky height, where no army could reach him and no siege could starve him out.

Arthur, Cei, and Bedwyr arrived at the coast as night fell. They could see a fire blazing on the summit of Mont Saint Michel, and on a lesser eminence some distance away in the sea, a smaller fire burned. Bedwyr was dispatched in a boat to discover who had lit the second fire. He landed on the smaller island and found an old woman lamenting loudly, drying her tears as she spoke to him. She begged him to flee, for the monster who inhabited the greater mount would rend him limb from limb and sup on his flesh. But yesterday, she said, she had been the nurse of the fair Helena, niece to Duke Hoel, who lay buried there beside her. The lady was no more. When the accursed giant had seized upon her, terror had so overcome her that her spirit had taken flight before the brute could do her further harm. She had died of fear, not of violence, and the giant had buried her on the lesser eminence where the nurse now kept vigil over her grave.

Bedivere returned with the dreadful news. Arthur, undismayed by what he had heard, waited until all were asleep and then set out with Cei and Bedwyr to scale the mount. They climbed the rugged height in darkness, guided by the fire on the summit, and found the giant sprawled beside the flames, his monstrous form cast in shadow and light. The battle was brief and brutal. The giant seized a mighty club and swung it at Arthur, but Arthur was quicker. He dodged the blow, seized the club, and hurled it back with such force that it struck the giant in the forehead. The monster fell dead, his skull shattered by his own weapon, and Arthur stood victorious over the body of the brute who had killed an innocent woman. But the victory was hollow. Helena was already dead, buried in the earth of the lesser eminence, beyond the reach of any rescue. A tomb was erected on Mont Saint Michel in her memory, and the place became a site of pilgrimage and mourning, the grave of a lady whose only crime was to be beautiful enough to attract a monster.

The story of Helena and the Giant of Mont Saint Michel became one of the most enduring episodes in the Arthurian tradition. Wace, writing his Roman de Brut around 1155, retold the tale with the name Eleine, and Layamon in his Brut of the late 12th to mid-13th century used the name Elaine. In the Vulgate Merlin, the victim's uncle is Lionel of Nanteuil rather than Hoel, and in the Norse Tristrams Saga the character is called Orsl's daughter, showing how the tale travelled and transformed across languages and cultures. In Thomas Malory's Le Morte d'Arthur of 1485, the lady is not Hoel's niece but his wife, the Duchess of Brittany, a change that may reflect Malory's desire to heighten the tragedy by making the victim a married woman of high rank rather than a young maiden. But in every version, the core of the story remains the same: a giant from Spain carries off a woman of the Breton court, Arthur kills the giant, and the rescue comes too late. The tale is a meditation on the limits of even the greatest hero's power, the reality that speed and strength can punish a crime but cannot undo it, and that the innocent die while the avenger is still mounting his horse.