For seven years the Greeks and Trojans have been at war following the Trojan prince Paris' abduction of Helen, the most beautiful woman in the world, from her Greek husband Menelaus.
The Greek army is encamped under the walls of Troy and, at the point at which the play begins, the war has reached stalemate. The Greeks are quarrelling amongst themselves. Achilles, their greatest champion, refuses to fight and has withdrawn to his tent with his lover, Patroclus.
Ulysses tries to entice Achilles back to the field by arousing his jealously against Ajax, a rival warrior, whom he announces as their new hero and opts to meet Hector, the Trojan champion, in single combat.
Equally at odds with themselves, the Trojans are debating the value of continuing the war merely for the sake of keeping Helen. Hector declares that she is not worth the lives she costs, but when his brother Troilus contends that honour demands they continue to fight for her, Hector is brought round to his point of view. Although the single combat between Ajax and Hector ends in a show of friendship, hostilities are resumed the following day.
Troilus, however, is much distracted from these military concerns by his love for Cressida, the daughter of Calchas, a Trojan who has defected to the Greek camp whilst leaving his daughter in Troy. The young lovers are eagerly assisted by Cressida's uncle Pandarus, who acts as their go-between. However, after only one night together they are parted when, in exchange for the captured general Antenor, Cressida is sent to join her father in the Greek camp. Almost immediately she betrays Troilus with the Greek Diomedes and, discovering this, Troilus is plunged into despair.
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Despite his sister Cassandra's prophecies of doom, Hector goes into battle and is treacherously murdered by Achilles, who has finally been roused into action by the death of Patroclus. With the fall of Troy certain, Troilus, disillusioned as a lover, assumes Hector's role as the Trojan champion and vows revenge on Achilles. The dying, disease-ridden Pandarus is left to end the play.