Several years into the Trojan War, the two sides have reached a stalemate. The Greeks have besieged the city of Troy but have not been able to breach the walls. They’re also facing dissent within their own ranks, led by the arrogant Achilles, who refuses to fight anymore.
Within the walls of Troy, a smaller conflict rages in the heart of Troilus, the youngest of King Priam’s sons, who is experiencing the pangs of unrequited love for a beautiful Trojan woman named Cressida. Pandarus, Cressida’s uncle and Troilus’s friend, has been trying to help, but convincing Cressida to give the prince a chance has been slow work—that is, until Cressida sees the valiant Troilus returning from the battlefield a few days later. When Pandarus points Troilus out to her through a window in her house, it’s clear that she’s impressed.
In the Greek camp, generals Agamemnon, Menelaus, Nestor, Ulysses, and Diomedes gather to discuss the state of the war and their forces. Disorder and dissent seem to be the order of the day. When Trojan lord Aeneas comes to issue a challenge clearly meant for Achilles (one-on-one combat with Trojan crown prince, Hector), Ulysses hatches a plan to rebuke Achilles’s arrogance by having another Greek warrior, Ajax, take up the challenge—and the chance for glory—instead. Although they give Achilles one last chance to rejoin the war effort, when he sends word by way of his friend Patroclus that he’s not interested, Ulysses puts his plan into motion.
Meanwhile, in Troy, Priam holds a war council to discuss whether it’s worth continuing the deadly war or whether they should just send Helen—whom his son Paris abducted from her Greek husband Menelaus—back to her own people. Ultimately, they decide that honor demands they fight to keep as valuable a commodity as the beautiful Helen.
Not long afterward, Pandarus asks Paris to cover Troilus’s absence from the palace so that he and Cressida can finally meet in person. Initially shy and reluctant, Cressida quickly reveals that she’s loved him for some time, but has held her tongue out of fear that he will value her less if he thinks her too easy a conquest. They go to bed together. But by the time they wake up in the morning, her father Calchas, who defected to the Greek side, has convinced Agamemnon to trade a valuable Trojan prisoner for her. Diomedes comes to Troy as an emissary to fetch her. Distraught, Troilus nevertheless hands her over without a fight. Diomedes takes an immediate liking to the beautiful Cressida.
The two sides declare a temporary truce so that Ajax and Hector can fight each other, and the Greeks welcome the Trojans as honored guests in their camp. The fight is inconclusive, however, since Hector and Ajax are cousins and Hector refuses to kill his own kin. When the bout has ended, Greeks and Trojans prepare to sit down to a shared feast in Agamemnon’s tent. But when Diomedes slips off in a different direction, Ulysses and Troilus, followed secretly by Thersites, a bitter and cynical Greek foot soldier, follow him to the tent where Cressida and Calchas are staying. The three men watch from the shadows as Diomedes presses Cressida to make good on the promise she made him (by implication, a promise to have sex with him). Although she wavers a little, she ultimately gives in to his suit, blaming the weakness and inconstancy of her female gender for her disloyalty.
Brokenhearted, Troilus initially doubts the evidence of his eyes, but Ulysses confirms all that he saw and heard. Troilus is thus cursing Cressida’s name when Aeneas fetches him back to Troy to prepare for the day’s battle.
In the city, Hector’s sister Cassandra and his wife Andromache try to convince him to stay home. Both have had dreams and visions of his imminent demise. But, because Achilles insulted him the previous day, he plans to fight regardless of the danger. Initially, Achilles continues in his refusal to fight (apparently, he promised the Trojan princess Polyxena, with whom he fell in love, not to), at least until Hector kills his best friend Patroclus. Then, furious, he takes up his weapons once again. Out of shape from months of idleness, however, he can’t best Hector. Instead, he and his men ambush the Trojan prince at the end of the day, when he is unarmed and unprepared. They murder him without giving him a chance to defend himself.
As the Trojans retreat behind the walls, the scope of their losses becomes clear. Pandarus meets Troilus at the gates. He’s ill and in pain, but Troilus curses him rather than offering compassion. As Troilus stalks away, Pandarus speaks directly to the audience, implicating them in the moral decay they’ve witnessed in the play and cursing them with painful deaths from venereal diseases.