The action begins at a tavern just outside of London, circa 1390, where a group of pilgrims have gathered in preparation for their journey to visit the shrine of St. Thomas Becket in Canterbury. The narrator, Chaucer, encounters them there and becomes one of their company. Chaucer describes all of the pilgrims in delightful, and often grotesque, detail.
The pilgrims go to dinner, during which the owner of the tavern, or Host, makes a proposal to the group: on the way to Canterbury, says the Host, each pilgrim will tell two tales, followed by two on the way back. The Host will accompany the group and serve as a judge of their tales. The pilgrim who tells the best tale wins a free dinner at the tavern at the journey's end. Should anyone question the Host's judgment, moreover, he has to foot the bill for the entire pilgrimage. The pilgrims, eager to have fun on their journey, quickly agree to the Host's proposal and swear oaths to abide by the rules of the game. After a bit of shut-eye, they ride out of Canterbury the next morning and the tale-telling begins.
Almost immediately, a pilgrim challenges the Host's authority. After the first tale, the Host asks the Monk to tell a tale, but the drunken Miller interrupts him and announces that he will speak next or leave the company. It's certainly not the last time the Host's orderly vision for the game is challenged: drunken pilgrims, mysterious strangers, and, most importantly, the conflicts between some of the members of the company threaten to derail the game at many points in the course of the journey.
The pilgrims tell lots of different kinds of tales on their journey: comedies and tragedies, romances and dirty stories, and sermons and saints' lives, to name a few. Some pilgrims tell stories where a character with another pilgrim's occupation is humiliated in the course of the tale, which leads to trouble. The Miller, for example, tells a tale about a carpenter whose wife not only commits adultery with a clerk, but humiliates him in front of the whole town. The real carpenter among the pilgrims takes this very personally, and proceeds to tell a tale where a miller suffers humiliation at the hands of some students. A similar rivalry occurs between the Friar and the Summoner. All the while, the Host alternates between trying to make peace between the pilgrims and creating more conflict with his gentle and not-so-gentle teasing of members of the party.
The Canterbury Tales end after only 24 tales, a far cry short of the planned 120. We never get to see the pilgrims reach Canterbury, nor do we learn who wins the competition. It's likely that Chaucer ran out of time or energy. He may have planned to revise the beginning of the frame story so that the 24 tales would seem complete. In any case, The Canterbury Tales as we know them end with the Parson's sermon on sin and repentance, followed by Chaucer's retraction.