Troilus and Criseyde is split into five separate books. In the first two, Troilus discovers and woos Criseyde. The third book is climatic, in which the couple celebrate their love. In the fourth book, they are separated. The fifth outlines the fate of both of them while apart.
Each book begins with a small poem, addressed to different Gods to offer good will for what is to come. The first book opens with a poem to a Fury, Tisiphone, as a prayer for the lovers who will soon be introduced. The poem also forewarns the reader of the "double sorrow" that Troilus will experience; thus we know from the beginning their love is doomed to fail. The setting is the Trojan War, inside the walls of Troy with the Greeks camped outside to siege them. The soothsayer Calchas foresees that the Greeks will take Troy. With this knowledge, he deserts the Trojans and joins the Greek camp. He leaves behind a daughter, Criseyde, who is now vulnerable as a young, unmarried maiden by herself in the city. She seeks the protection of Hector, a Trojan prince and son of King Priam. Troilus is the brother of Hector and serves as a soldier in the Trojan army.
Troilus is in the temple of Pallas Athena with his knights. He mocks them for being lovesick for the women there, and speaks about how those who are in love are foolish. He is then struck by the God of Love, and sees Criseyde. He falls instantly in love with her, and spends the following days in agony, not knowing how to deal with his lovesickness.
Criseyde’s uncle, Pandarus, counsels Troilus and encourages him to repent and be humbled by the God of Love. They begin to think of a plan for Troilus to tell Criseyde how he feels.
The second book begins with a poem to a muse of history, Clio. The author prays that she will help him to write the book well, and for it to rhyme.
Pandarus delivers a speech on how he is unworthy of love, which encourages him to help Troilus and Criseyde unite. Pandarus goes to see his niece and teases her to cheer up. After a long-winded verbal exchange, Pandarus reveals Troilus’ feelings towards her. Pandarus urges Criseyde to consider Troilus as a love interest, and manipulates her by claiming both he and Troilus will commit suicide if she does not commit her love to him. She is reminded that she is no longer young and that there is a pressure for her to marry soon.
The first window scene occurs. Troilus has returned from a battle and is parading down the street, both proud and embarrassed from the attention. From her window, Criseyde can see him clearly. Alone, she considers the benefits and downfalls of having a lover. She admits that she is vulnerable and in need of protection. Yet, she is also a widow and taking Troilus would mean a loss of freedom. Criseyde goes out to her garden with her ladies, and hears Antigone singing a song of love. That night, she goes to bed and dreams of a large white eagle (representative of Troilus) painlessly taking her heart and replacing it with his own.
Then begins the exchange of letters that starts the love affair. The process is constantly watched over by Pandarus, who urges each party to be forward in their letters, and to reply immediately. Firstly, Troilus writes a passionate letter to Criseyde, which he seals with his tears; the suggestion of Pandarus. Pandarus delivers it to Criseyde, and urges her to reply straight away, asking to meet with Troilus. She is reluctant yet still writes a letter. More letters are exchanged between the two and a relationship begins to form.
Pandarus visits Deiphebus to try and arrange for Troilus and Criseyde to meet. They decide to arrange a meeting at Deiphebus’ house and Pandarus invites Criseyde. Pandarus has already brought Troilus to the house, and has told him to go to bed under the pretense of an illness. When Criseyde arrives at the house, Panarus takes her to Troilus’ chamber. The two lovers meet face-to-face for the first time.
The poem that begins Book 3 is addressed to Venus, goddess of Love. It praises her power and asks her to bless Troilus and Criseyde’s love.
This book returns to where Book 2 left off, with Criseyde being brought to Troilus’s chamber in Deiphebus’ house. Troilus passionately speaks to Criseyde and asks if he can serve her; Criseyde agrees, somewhat ambivalently. Both Pandarus and Troilus are elated. Later, Pandarus talks to Troilus seriously about treating his niece with honor. After this occasion, a length of time passes where the couple exchange letters and meet several times.
On an evening where rain is threatening to fall, Pandarus invites Criseyde to his house for dinner. She attends, and while they are dining, Troilus is hidden and observing them. The rain worsens after supper, and Pandarus convinces Criseyde that she cannot return home in such weather, and persuades her to stay the night. Pandarus leads Criseyde to a private chamber. Pandarus tells Criseyde a false story about Troilus being jealous over rumors of Criseyde's other potential suitor. Pandarus insists she talk to Troilus directly to clear up the matter and brings him in. Criseyde weeps and reassures Troilus it is not true. In the heightened emotion, the couple agree to put the conflict behind them and they embrace. They exchange marital-like vows before spending an intimate night together, parting at dawn.
Pandarus warns Troilus to be cautious in the relationship, reminding him how quickly one's fate can change depending on the position of Fortune's wheel. The couple spend more nights together and both live in a blissful state of happiness. Invigorated by his new love, Troilus becomes a more virtuous, chivalrous soldier.
Book 4 opens with a poem to Mars, the God of War, and the three furies. Instead of condemning Criseyde for her future, the author urges the reader to have mercy upon her.
Criseyde’s father Calchas asks for a treaty from the Trojan camp to exchange Criseyde for Antenor, a Trojan lord captured by the Greeks. It is agreed on by both sides to do the exchange. Upon hearing the news, Troilus laments that Fortune has always been against him and goes into despair.
Troilus discusses the exchange with Pandarus, and they muse on what to do. Pandarus suggests that Troilus elope with Criseyde, but Troilus refuses, explaining that as a chivalrous soldier, he cannot do anything so dishonorable.
Criseyde is also very saddened to hear of the exchange. She is visited by her women at the palace and must hide her sorrow at leaving Troy. When she is alone, she allows herself to weep. Pandarus visits her and informs her of Troilus’ sorrow. Meanwhile, Troilus is in a temple, meditating on predestination and whether human choice is a factor in one’s fortune. Pandarus appears, and reassures him that all will be well, urging him to go to Criseyde.
Troilus visits Criseyde and they emotionally discuss her leaving Troy. She rejects the idea of running away together, saying they would regret it later. Instead, she reassures Troilus that she will deceive her father and return to him in Troy in 10 days time. Troilus gives Criseyde a special brooch to remember him by. Troilus leaves her in the morning with a sense of dread.
Criseyde is exchanged for Antenor and she joins the Greek camp. Immediately, the Greek warrior Diomede offers to protect Criseyde from any harm.
Troilus spends his days pining for Criseyde and feeling much anticipation for when she will return. He and Pandarus visit King Sarpedon’s villa, a place of excess and merriment, yet Troilus cannot enjoy it. On the tenth day after her departure, Criseyde does not return to Troy as promised. She writes to Troilus, explaining it is too difficult for her to leave under her father's watch. Although she deeply misses Troilus, she is already letting herself be wooed by Diomede. She decides that she is in need of protection, and accepts Diomede as her lover, giving him the brooch that Troilus gifted her. She experiences much remorse but feels it is the best thing to do.
Troilus keeps waiting for Criseyde’s return, but eventually sees she has broken her promise. He dreams of Criseyde embracing a boar. He writes Criseyde a heartfelt letter, asking why she has not come. She writes a reply, but it is vague and short. Troilus visits his sister Cassandra to unravel the meaning of his dream, and she describes the boar as a new lover. Troilus refuses to believe Criseyde’s betrayal is true. Troilus continues to write to her, but her replies are consistently short and uninterested. The Trojans capture one of Diomede’s garments, and Troilus discovers upon it the brooch he gave to Criseyde. This confirms the affair.
Troilus moans about bad fortune and laments being betrayed. For the first time, Pandarus has nothing to say but that he is sorry. The narrator briefly describes Troilus’ death in battle, where he is killed by Achilles. He ascends to the eighth sphere of heaven where he realizes the vanity of worldly affairs and laughs at those mourning for him. The narrator discusses the transience of life and finishes by asking for the protection of the Trinity and Christ’s mercy.