By introducing John first, and Nicholas and Alisoun mainly as they relate to John, the narrator makes us think that he is the protagonist. He seems to be saying: this is a story about a carpenter and, secondarily, his wife and boarder. And given that almost all the action happens in John's house and concerns his wife, it's perhaps not unreasonable to accept John in this role. On the other hand, almost immediately the focus of the plot shifts from John to the affair between Nicholas and Alisoun, and it becomes Nicholas's concerns and motivations that drive the plot from that point forward. Hence, the story's initial presentation of John as the protagonist may be something of a bait-and-switch. In the end, John is perhaps nothing more than a convenient epicenter around which to plot the action.