A miller named Symkyn lives on some property by a bridge not far from the town of Cambridge. (A miller is a person who grinds corn and grain into flour.) He likes to fight, carries multiple weapons, and enjoys wrestling. Most people in the town avoid conflict with him, even though he regularly cheats his customers by stealing corn from them or "padding" their sacks of flour with less-expensive substances. Symkyn has married the illegitimate daughter of a local cleric, a woman who's proud because of her expensive upbringing in a nunnery.
When the manciple of a school in Canterbury that regularly grinds its corn and wheat with Symkyn gets sick, the miller takes the opportunity to cheat the school even more than usual. Two students there, Aleyn and John, ask their headmaster to allow them to go have the corn ground the next time it needs grinding, convinced they can prevent the miller from cheating them. The headmaster agrees, and the two set out on their journey by horseback.
When they arrive at the mill, Aleyn and John tell Symkyn that they will watch the corn being ground. Symkyn realizes they want to watch to keep from being cheated and takes their vigilance as a challenge. He unties Aleyn and John's horse. When they find it missing, they have to spend all day chasing it in a nearby field, giving Symkyn time to steal flour from them, which he has his wife bake into a cake and hide.
Having wasted the whole day horse-catching, Aleyn and John decide to stay the night at Symkyn's house. That evening everyone beds down in the same room. Aleyn decides to use the close proximity of Symkyn's daughter, Malyne, to have sex with her. The clerk figures he's owed this because Symkyn cheated him out of the corn. John does the same with Symkyn's wife, moving the cradle at the foot of her bed to his so she will climb in with him by mistake. At dawn, when Aleyn tries to return to his bed, the misplacement of the cradle causes him to crawl into bed with Symkyn by accident. Thinking he's John, Aleyn boasts to Symkyn that he had sex with the miller's daughter all night.
Enraged, Symkyn rises out of bed and punches Aleyn in the nose, then tumbles onto the bed where John and his wife are sleeping. When his wife wakes up, she tries to help her husband by hitting the clerks with a staff, but mistakes her husband's bald skull for the white caps the clerks wear, and knocks him over the head instead. Aleyn and John beat Symkyn up, then hightail it off of his property, picking up the cake made of their stolen flour, the location of which Malyne has revealed to Aleyn, on the way.