This plot analysis relies upon our acceptance of Nicholas as the "hero" and Alisoun as the "heroine" in the story, character role identifications that are debatable, as we discuss in the "Character Roles" section.
The winsome young heroine Alisoun is married to a much older man, a carpenter named John. Because he is so much older, John is not an acceptable sexual partner for Alisoun, or so the story suggests. Things are not as they should be, but hope exists in the form of a young boarder named Nicholas, a much more appropriate lover for Alisoun who very much desires to hook up with her.
Alisoun and Nicholas start an affair but cannot be openly together or spend the whole night in one another's arms. According to Alisoun, John is so jealous he will hurt her if he finds out. Alisoun and Nicholas's affair is complicated, moreover, by the presence of another would-be lover, Absolon, who courts Alisoun incessantly.
Alisoun and Nicholas are finally able to spend the night together after Nicholas tricks John into sleeping in a tub hanging from his rafters. The culmination of this ruse, in which John takes a nasty tumble from the rafters only to be called crazy by Nicholas and Alisoun in front of his neighbors, renders John prone and powerless, hence removing him as the pressure of darkness. Absolon is cured of his love-sickness by Alisoun's nasty trick on him, removing another obstacle to our hero and heroine's union.