For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
John and Aleyn travel to the mill to have their corn ground, but fail to prevent Symkyn from cheating them when they must catch their horse instead. The two decide to spend the night at the miller's house, and end up sharing a room with the whole family.
Act II
Seeking recompense for their stolen corn and the rough day they've had, John and Aleyn use their close proximity to all of Symkyn's family to debauch his wife and daughter. Trying to make it back to his own bed in the morning, but not realizing that John has moved the cradle, Aleyn crawls into bed with Symkyn by mistake. Thinking he's talking to John, Aleyn boasts to Symkyn that he had sex with Malyne all night.
Act III
John rises in a rage, and punches Aleyn in the nose. Symkyn tumbles into the bed where his wife and John are sleeping, waking them. Symkyn's wife grabs a staff and hits her husband over the head by mistake. After beating Symkyn up, John and Aleyn flee his property, picking up their stolen corn on the way out.