The description we get of John's domestic situation perfectly sets us up for the conflict that's to follow. We learn not only that Alisoun is much younger than John and very beautiful (from a medieval point of view, a clear indication that she's likely to cheat on him), but also that John boards a young man in the house. You can pretty much guess what's coming.
Obviously, Alisoun's role as Nicholas's lover conflicts with her role as John's wife. And Absolon's courtship of Alisoun is unwelcome, since she already has both a lover and a husband. This second plot strand will be wound into the first with great effect at the conclusion.
These are the conflicts that drive the remainder of the plot. Alisoun's caution prompts Nicholas to trick John into believing he needs to sleep in a tub hanging from his rafters, thereby leaving Nicholas and Alisoun to frolic undisturbed. And Alisoun's repeated rejection of Absolon's seduction attempts only seems to spur him on.
This is the moment when the set-up comes to fruition and the complications are addressed. With John in the next room while Nicholas and Alisoun frolic and Absolon on his way to the house, it's the point at which the potential for conflict is at its highest.
Tensions are high: will John wise up to the trick and catch Nicholas and Alisoun in bed together? How will Nicholas handle Absolon's attentions to Alisoun? And how will Absolon react to his humiliation? Will John ever get out of that tub? This is the moment of suspense, the point at which many more questions are raised than answered.
Here we get answers to the questions we were wondering about in the suspense stage. John fails to wise up, Nicholas handles Absolon's attentions to Alisoun by continuing the joke she started, Absolon reacts badly to his humiliation, and John does get out of that tub, although painfully.
Final questions are tied up here. You may have been wondering if John will figure out he's been had once he sees Nicholas and Alisoun together and realizes there's no flood. If he does, we never get a chance to hear about it, because his protests are drowned out by Alisoun and Nicholas's insistence that he's crazy. Their success in convincing the neighbors of John's insanity rules out any possibility for John to make a case against Nicholas, for no one will believe anything he says from now on. The possibility of conflict averted, the story is over.