The Duke of Austria, being a rather over-confident man, assumes that he is childless because his wife is unable to conceive. It doesn't seem to occur to him that the problem might lie with him and so he threatens to divorce her if she does not become pregnant soon. As she is relaxing in the orchard one morning, she sees a figure she believes to be her husband and makes love with him underneath a big apple tree. However, she has been deceived, and in fact has been accosted by an anonymous impostor who has been cast in the image of her husband by Merlin. The child she is now carrying is Merlin's half brother.
The child is born without incident but as he grows up he seems to have a fiendish side to him. He is a difficult baby and several nannies and nurses end up dead as a result of his actions. He tears off the Duchess's nipple on the one occasion she tries to breast feed him. Once he is old enough to go on a hunt he becomes obsession with the chase and the kill, but soon, hunting animals does not satisfy him. He goes about the countryside brandishing a large sword. He rapes the nuns at a local convent and burns down the convent with his victims locked inside. The Duke, horrified to the depths of his soul by what his son has become, dies of shame.
After his father dies, Sir Gowther takes over the mantle of Duke of Austria, but there are mutterings about the identity of his true father, and he goes to his mother to find out if what he has been told is actually true. He threatens to impale her through with his sword if she lies to him and so she reluctantly admits the truth, but a strange change comes about her son as he lays down the sword and decides to make a pilgrimage to Rome where he shall ask the Pope for absolution.
The Pope gives Sir Gowther an audience and tells him that as a penance for his sins he may not speak and that he can only swallow food that has first been chewed by a dog. He is taken into the court of the Emperor of Germany and lives as the Fool under the table with the dogs there. The Emperor has a daughter, and she is mute too. A Sultan comes to claim her as his bride, but the Emperor does not want to let her go, which causes a war to break out between their nations. Sir Gowther, now living as Hob the Fool, prays hard that he might help the Emperor defend his daughter and his lands from the heathens attacking them. Magically a horse and armor appears outside his room, three times, and he rides with the Emperor's army unrecognized, disguised first as the black knight, then the red knight and lastly as the white knight. He is unbeatable and unbeaten in battle. He beheads the Sultan during a battle. Although nobody can figure out who these strange and colorful knights are, the Emperor's daughter is privy to the truth, but as she is mute she cannot share the secret with anyone.
When the Emperor's daughter sees Sir Gowther injured on the field on the final day of battle, she is so distraught that she falls from her tower. The Pope himself comes from Rome to bury her but as the funeral is about to start she awakens. Able suddenly to speak, she tells the gathered mourners that God has forgiven Sir Gowther all his sins, and he may also speak again, just as she can.
Sir Gowther and the Emperor's daughter are married, and Sir Gowther becomes the next Emperor of Germany after his wife's father passes away. He builds an abbey and attaches it to a convent so that all can pray for the souls of the nuns whom he burned alive. After Sir Gowther's death, there are many miracles seen by those paying their respects at his tomb.
