The beginning of this book laments that it will now tell of how Troilus lost Fortune’s favor. Personifying Fortune as a feminine goddess, Chaucer writes of how Fortune always tricks those who she seems to be favoring the most, turning their happiness into betrayal. He relates that Troilus will lose Criseyde, while Diomede will be elevated on Fortune’s Wheel to a higher state. Chaucer invokes the Erinyes, also known as the furies, Greek goddesses of revenge. He also asks for help from Mars, the god of war, to help him write this book and relate these events.

During the war with the Greeks, several Trojan warriors are captured. Both sides decide to arrange a prisoner exchange, but there are more Trojan prisoners than there are Greeks. Calkas goes to the Greek leaders and begs them to ask for his daughter, Criseyde, since he was unable to take her with him when he fled the city. He reminds them of how he has helped their cause with his predictions and left behind all his property to join them. The Greeks agree and request that Criseyde be traded in exchange for the Trojan Antenor.