Galehaut was born of a union between the Beautiful Giantess, a figure of such stature that her son inherited both her size and her nobility, and Brunor, the evil knight who held the Castle of Tears on the Giant's Isle, a fortress with customs so cruel that they drove Galehaut from his father's house to seek his own kingdom. He was not a giant like his mother but a half-giant, a man of enormous height and strength who moved through the world with the grace and courtesy of the finest knight in Christendom. He conquered his first kingdom, Sorelois, by defeating and killing its king, Gloier, but instead of destroying Gloier's house he took the orphaned daughter under his protection, a gesture that revealed the generosity that would define his character. From Sorelois he marched outward, conquering twenty-nine more kingdoms, including North Wales and the lands beyond the borders of Galone, until his banner flew over thirty realms and his name was known from one end of the known world to the other.

Yet Galehaut refused to be crowned. He would not call himself king, though he ruled more territory than most monarchs ever dreamed of, because he had sworn that he would not accept a crown until he had conquered Logres, the kingdom of King Arthur, the realm that stood at the centre of the Arthurian world and that no foreign army had ever subdued. He gathered his forces, summoned his vassals, and marched on Arthur's lands, laying siege to the territory of Selice, which lay under Arthur's protection. Arthur, caught unprepared, could not gather his full army in time, and it seemed that Logres itself would fall. Only two knights stood between Galehaut and total victory: Gawain, the eldest son of King Lot and the most renowned knight of the Round Table, and a mysterious warrior known only as the Red Knight, who fought with a skill that seemed superhuman. Galehaut's advance was checked, but not stopped. Rather than press his advantage and crush Arthur with an incomplete victory, he did something unprecedented: he gave Arthur a year to gather his entire army, promising to return when the king was at full strength.

A year later, the two armies met again on the field before Tintagel. Galehaut's forces outnumbered Arthur's so completely that victory was certain. The battle began, and once again a mysterious knight fought on Arthur's side, this time called the Black Knight, whose deeds were so extraordinary that even Gawain, watching from the field, was struck with wonder. The Black Knight cut through Galehaut's ranks like a scythe through wheat, unhorsing knight after knight, turning the tide of battle by sheer force of arms. Galehaut, watching from his command post, felt something he had never felt before: not fear, not anger, but a kind of awe that bordered on love. He had spent his life conquering men, but he had never seen a man like this. When the battle ended and the Black Knight prepared to ride away, Galehaut stopped him. He begged the stranger to spend the night with him, to lodge in his tent and share his company. The Black Knight agreed, but on one condition: that Galehaut surrender to Arthur the following day. Galehaut, who had been on the verge of conquering the last kingdom he needed for his crown, agreed without hesitation. He surrendered to Arthur, made peace with the king, and took his place at the Round Table, all for the sake of a single night's companionship with a man whose name he did not yet know.

That night changed everything. The Black Knight revealed himself to be Lancelot, the son of King Ban and the foster-son of the Lady of the Lake, the greatest warrior in the world and the man who would one day become the champion of the Round Table. Galehaut had surrendered thirty kingdoms' worth of ambition for the friendship of one man, and he never regretted it. The two became inseparable, riding together on adventures, sharing tents and meals, fighting side by side in every campaign. Galehaut joined the Knights of the Round Table, not for glory but to remain near Lancelot. When Lancelot fell in love with Queen Guinevere, it was Galehaut who discovered the secret, and Galehaut who used his influence with the Lady of Malahaut, a noblewoman who had herself loved Lancelot, to arrange the first kiss between the queen and the knight. He gave way before Guinevere, yielding Lancelot to her as he had yielded his victory to Arthur, not from weakness but from a generosity of spirit that recognised where Lancelot's heart truly belonged.

Galehaut's greatest service to his friend came during the affair of the False Guinevere, when an imposter who resembled the queen exactly appeared at Arthur's court and bewitched the king into believing she was his true wife. The real Guinevere was condemned to death, and only Lancelot's challenge to three knights from Camelides saved her. Galehaut offered Guinevere refuge in his own kingdom of Sorelois, sheltering the queen and her lover in his castle until the deception could be exposed and Arthur restored to his senses. It was an act of treason against his king, but Galehaut did not hesitate. His loyalty to Lancelot outweighed every other bond.

The end came through a series of cruel misunderstandings. Lancelot was captured by Morgan le Fay, the sorceress queen who was his own half-sister, and imprisoned in her castle. Then, after escaping, he wandered into the wilderness in a fit of madness, living as a naked wild man, eating raw meat and sleeping in ditches. Word reached the Round Table that Lancelot was dying, and then that he was dead. Galehaut, who had learned from his clerk Helyes of Thoulouse that his own life could be prolonged only by keeping Lancelot near him, refused to believe the worst. He set out on a quest to find his friend, searching from castle to castle, forest to forest, but the rumours grew stronger and the silence more terrible. At last, convinced that Lancelot had died and that he had not been there to save him, Galehaut returned to his chamber, bolted the door, and refused all food and drink. He would not see Guinevere, who came to comfort him. He would not see Gawain, who begged for entry. For ten days he fasted, wasting away until a wound broke open and a strange illness seized him simultaneously. When Gawain and his men finally broke down the door with a wooden ram, they found Galehaut dead on the floor, his hand outstretched as if reaching for a friend who was not there.

Lancelot, who had been rescued by a hermit and nursed back to health, returned to court too late. He found Galehaut's body laid out in a religious house and carried it to Joyous Gard, the castle he had won from its previous lord and renamed in honour of his happiness. There he buried his friend in a tomb built to consecrate their companionship, and when Lancelot himself died years later, he was laid to rest in the same tomb, beside the man who had given up everything for him. Long after his death, Galehaut's name became a byword for the power of friendship and the tragedy of love. Dante Alighieri, writing in the Inferno, made Galehaut the book that Paolo and Francesca had been reading when they yielded to their adulterous passion, a detail that forever linked his name with the intermediary who brings lovers together. Giovanni Boccaccio used his name as the subtitle of the Decameron, calling it "Il Principe Galeotto" - Prince Galehaut - the one who brings people together through storytelling. In Spanish, galeoto became an archaic word for a pimp, a sad diminution of a name that had once meant nobility and sacrifice.