King Horn is part of "The matter of England", a term coined by literary scholars grouping medieval literature that discusses anything related to England, either historically, personally or culturally. The stories protagonist, Horn, has his life overturned when his father's kingdom is invaded by Saracens, his father is killed and he and his friends are captured. The hero's mother turns slightly crazy and retreats into a cave all by herself.
The main troupe consists of Horn and his two main companions, Athulf and Fikenhild. Their ill fate turn when Horn's outer appearance so impresses the leader of the Saracens that he sets them free, lets them float off in a boat. The troupe's next port is the country of Westernesse. They become part of the royal court of King Ailmar and spent some years there. Horn, now finally adult, starts a secret affair with the king's daughter Rymenhild. Their love for each other is so great that they soon decide to marry each other and Rymenhild presents her lover with a ring of betrothal. This is discovered by a jealous Fikenhild, who betrays his friend's trust and reveals the marriage plans to Ailmar. The king is enraged and forces Horn to leave his country or be killed. Before he leaves, he promises Rymenhild to be back in seven years or she should marry someone else.
The hero's journey so continues and brings him to the country of Ireland and incognito enters the court of king Thurston. He soon becomes the shield brother of the king's sons and together they slay the Saracens that were responsible for the death of Horn's father. While sad at their death, Thurston is impressed with his new knight's bravery and offers him to make him his heir by giving him his daughter's hand in marriage. Remembering his promise to Rymenhild, Horn refuses and asks the king to give him seven years to make his decision to marry Reynhild, the king's daughter. The years pass and Rymenhild gets engaged to a hateful person, the king Modi of Reynes. She desperately tries to reach Horn by letter, but only one reaches him. His answer gets lost at sea.
Being determined to save his love from her marriage, Horn finally reveals his true identity to his liege Thurston and offers his best friend Athulf as a potential groom for Reynhild. He then leaves for Westernesse. Arriving there he discovers that the marriage has already taken place. Aggrieved he assumes that her love must have not been strong enough and decides to play a trick to prove her love. He disguises as a palmer with blackened skin and sends the ring she gave him as a present to the new queen. Rymenhild is distraught to receive her ring back from an old palmer. She requests to see him and then asks where he got the ring. Horn answers that he got it from Horn, who died. Rymenhild is beyond sad and tries to take her own life. In this moment Horn reveals his true identity and a happy reunion ensues.
With the help of his Irish entourage, Horn slays the whole court of king Modi and then forces Ailmar to let Rymenhild marry him. At the wedding ceremony he reveals his true identity and promises that they will live happily ever after once his own homelands are safe. He takes leave of his new bride and sails for Suddene where he defeats the invaders and is finally crowned king. Back in Westernesse, his old friend Fikenhild, still jealous, claims that he heard word that Horn has died and asks for Rymenhild's hand in marriage. He imprisons Rymenhild in a newly built castle by the sea. Having heard of this atrocity, Horn and his band of loyal followers return to Westernesse, disguise themselves as a troupe of players and march on the castle. Their jolly play causes Rymenhild to faint, as her heart has known only sorrow. Horn is saddened by this and finally reveals his true identity again, slays his old friend Fikenhild and claims Rymenhild for himself.
The end of the poem is used to finish of the story and give a happy ending to everyone. The loyal Athulf gets to marry Reynhild of Ireland, and the loyal stewards that helped Horn get to become kings of the new vacant kingdoms. The royal pair of Horn and Rymenhild return to Suddene and rule over a happy and eternal kingdom.
