In “The Knight's Tale,” Chaucer employs classicism in both the story's content and style. Classicism is essentially looking back to classical antiquity—the Greek and Roman worlds—and attempting to imitate it in some way.
Chaucer sets the Knight's story in ancient Greece, specifically in the city of Athens. The duke Theseus (who in mythology is the founding king of Athens) is at war with the city of Thebes, and he captures two wounded Theban soldiers, Palamon and Arcite. The two end up feuding over Theseus's sister-in-law, Emelye (sister of the queen Hippolyta, who is also familiar from mythology).
Eventually, Theseus declares that the two will fight for Emelye's hand, and both Palamon and Arcite show up with armies to back them. Then the gods get involved. Palamon relies on Venus and Arcite on Mars, and Chaucer offers scenes within the temples of each as the rivals pray to their chosen patrons. Emelye also reaches for divine aid and appeals to Diana. The gods determine the victory, for although Arcite seems to win, Saturn steps in with an earthquake, and the victor falls from his horse and dies. Palamon eventually wins Emelye's hand after Theseus gives them a lesson about fate.
We can see, then, the classical elements in the content of the story, but the tale also gives a nod to classical style, for it imitates a classical epic in its high style and formal language (as opposed to the rollicking informality of some of the other tales), its epic question (the role of fate and free will in human life), its epic games (the contest between Palamon and Arcite), and its epic hero (Theseus). While Chaucer does not write an actual classical epic, these elements at least suggest that the form was in his mind and influenced his decisions as he composed the tale.