Horn, the fairest youth ever born, is bright as glass and white as a flower; his color is rose-red, and he has no equal in any kingdom. When Horn is fifteen years old, his father, King Murry of Suddene Isle of Man, is killed by invading Saracens. His mother, Queen Godhild, finds refuge under a rock, where she prays for Horn’s safety. Because of Horn’s fairness, the Saracens spare him, setting him adrift with twelve companions, among them his brother Athulf and the wicked Fikenhild, on a ship that they expect will sink. The youths land safely on the shore of Westernesse, where good King Aylmar receives them kindly and takes a special liking to Horn. Aylmar’s daughter, Rymenhild, is also attracted to Horn and asks the steward, Athelbrus, who is in charge of Horn’s instruction, to bring him to her room. Disturbed at this command, Athelbrus brings Athulf instead. Mistaking him for Horn, Rymenhild tells Athulf that she loves him. When she discovers that Athelbrus tricked her, she threatens to have him hanged, whereupon Athelbrus brings Horn to her. Rymenhild asks him to marry her, but Horn refuses, saying that he is a foundling and unworthy. At this rebuff, Rymenhild falls in a swoon. Horn takes her in his arms, kisses her, and asks her to have her father make him a knight so that he might marry her.

King Aylmar knights Horn and permits him to knight his companions. As soon as Horn is knighted, Rymenhild wants him to marry her; but Horn says that he must first prove his merit as a knight. Rymenhild gives him a ring engraved with her name and tells him that if he looks at the ring and thinks of her, he will overcome all enemies. On a handsome black steed, Horn sets forth on his quest. He quickly finds and slays at least one hundred Saracens. The next day, Rymenhild tells him that she dreamed that a great fish escaped from her net. The significance of her distressing dream is clear when Fikenhild, envious of Horn, tells King Aylmar that Horn is planning to kill him and marry Rymenhild. He says that Horn is at the moment in bed with Rymenhild. Aylmar, rushing into his daughter’s chamber, finds Horn embracing Rymenhild. The king orders Horn to leave the castle. Before departing, Horn instructs Athulf to guard Rymenhild. He tells Rymenhild that he expects to be back in seven years; if he does not return by that time, she is to take another husband.

Horn goes to Ireland, where he meets two princes, Harild and Berild. He tells them that his name is Cutberd, and they take him to their father, King Thurston. The time is Christmas. Soon a giant comes from heathendom to offer a challenge from paynims who arrived in the land. One of them offers to fight any three of the Irish knights. The king appoints his sons, Harild and Berild, as well as Cutberd. Cutberd offers to take on the challenger alone. Having fought with Cutberd, the champion says that he encountered only one man his equal, King Murry of Suddene. Shuddering, Horn realizes that he is facing his father’s murderer. He looks on his ring, thinks of Rymenhild, and smites the champion through the heart. The paynims turn to run to the boat, but Horn and his companions follow and kill them all. Harild and Berild are killed in the fighting. Thurston offers his daughter Reynild in marriage to Horn and plans to make the young knight heir to the throne. Horn replies that he will serve the king for seven years. At the end of that time, if he wants his reward, including the princess for his wife, he will ask for it.

Back in Westernesse, Rymenhild hears nothing of Horn. King Modi of Reynes (Turness in northern Lancashire), Horn’s enemy, wants to marry her. She sends a messenger to find Horn. The messenger succeeds in his mission, but on his return he drowns and his body washes up at Rymenhild’s door. Horn, meanwhile, asks King Thurston to help him regain Rymenhild. In return, he promises his brother Athulf as husband for Reynild. Thurston gives him a ship, but when Horn arrives in Westernesse he finds Rymenhild’s wedding to King Modi in progress. A palmer tells him that the bride wept. Horn changes clothes with the palmer, disguises his features with dirt, and goes to the wedding feast, where he asks the bride for wine. Rymenhild gives him wine in a bowl as if he is a thirsty beggar. Horn refuses, saying he is a fisherman who came to see if the net he set seven years ago took a fish. He says that he wants to drink to Horn from horn. Rymenhild gives him wine in a drinking horn, and Horn drops in it the ring that Rymenhild gave him. Rymenhild sees the ring and asks if Horn is dead. Horn replies that he died aboard ship after asking him to tell her of his death. Rymenhild throws herself on her bed and prepares to kill herself with the knife she hid there to kill both Modi and herself that night. Horn wipes the dirt from his face and tells her that he is Horn, her true lover. Rymenhild runs to tell Athulf, who jumps for joy. Returning to the wedding party with his Irish warriors, Horn kills King Modi and his followers. After convincing King Aylmar that Fikenhild slandered him, Horn tells the king that he will return to Suddene and regain his kingdom, then marry Rymenhild. Horn recovers his kingdom from the Saracens and finds that his mother is still alive.

While Horn is gone, Fikenhild, through bribery and by intimidating the king, is able to carry Rymenhild off to his castle. Warned of this in a dream, Horn returns to Westernesse, only to hear that Fikenhild married Rymenhild. Disguised as harpers, Horn and his men gain access to Fikenhild’s castle, where he kills Fikenhild, hacks him to pieces, and rescues Rymenhild. He makes Athelbrus, the good steward, king of Reynes in place of Modi, and he takes Athulf to Ireland to marry Reynild. Then Horn takes Rymenhild to Suddene and there makes her his queen.