The play opens with the entrance of the Prologue, an actor dressed as a soldier, who gives us the background to the story, which takes place during the Trojan War. Immortalized in Greek mythology and Homer's Iliad, the war occurs because a Trojan prince, Paris, steals the beautiful Helen from her husband, King Menelaus of Sparta, and carries her home to Troy with him. In response, Menelaus gathers all of his fellow Greek kings, and together they sail to Troy, hoping to capture the city and reclaim Helen. The play's story, the Prologue informs us, begins in the middle of the conflict, after the siege of Troy has been ongoing for seven years.
Within the walls of Troy, Prince Troilus complains to Pandarus that he is unable to fight because of heartache—he is desperately in love with Pandarus's niece, Cressida, and praises her beauty to the skies. (Cressida's father, a Trojan priest, has betrayed his city and gone over to the Greeks.) Pandarus complains that he has been doing his best to further Troilus's pursuit of his niece, and that he has received small thanks for his labors. After he departs, Troilus remarks that Pandarus has been growing irritable lately, but that in order to win Cressida, he must continue to work through her uncle. As he ponders, the Trojan commander Aeneas dashes in, bringing word from the battlefield that Paris has been wounded in combat with Menelaus. As the noise of battle comes in from offstage, Troilus agrees to join his Trojan comrades on the field.
In another part of the city, Cressida converses with her servant, who recounts how a Greek warrior named Ajax, a valiant but stupid man, managed to overcome the great Trojan prince Hector the previous day, and that Hector is fighting furiously because of this defeat. (Ajax, despite being Greek, is Hector's nephew.) Cressida is joined by Pandarus, and they discuss the Trojan princes, with Pandarus taking the unlikely position that Troilus is a greater man than Hector. As they converse, several Trojan lords pass by them returning from battle, including Antenor, Aeneas, Hector, and Paris; Pandarus praises each one, but tells his niece that none of them can match Troilus. Finally, Troilus passes, and Pandarus crows that "had I a sister were a grace, or a daughter a goddess, he should take his choice. O admirable man!" (I.ii.244-46). He then leaves Cressida, promising to bring a token from Troilus. Alone, Cressida remarks that while she returns Troilus's feelings, she is holding him off; she is enjoying his pursuit of her.
The appearance of the Prologue, with his brief account of how the Greek kings have sailed to Troy to sack the city and recapture the stolen Helen, is a reminder of the dependence of this play on classical mythology. Also, by its very brevity, it indicates the high degree of familiarity that Shakespeare's audience would have had with this mythology, since the Prologue does not bother to give more than a few background details before leaping into the play itself. Before the action begins, however, the audience is offered this thought: "Like or find fault; do as your pleasures are; / Now good or bad, 'tis but the chance of war" (Prologue, 30- 31). This idea—that "the chance of war" is stronger than "good or bad"—dominates a play in which heroism and nobility are consistently undermined by baser qualities, a play in which lust and brutality rule the day. Indeed, while the setting—the Trojan War—might lead the audience to expect a traditional heroic epic, the Prologue's admonition foreshadows the fact that, in Troilus and Cressida, Shakespeare has actually constructed a deliberately anti- heroic story.
Troilus, in these opening scenes, is a fairly typical lovesick Shakespearean hero: his plaintive sighs and mopings call to mind figures like Romeo in Romeo and Juliet and Orsino in Twelfth Night. Troilus's first conversation with Pandarus is important primarily because it establishes two key aspects of the romance: Pandarus's role as go-between, and the problem of Cressida's father, who is mentioned only briefly here but will later be the cause of the two lovers' separation. The first scene ends with Troilus being called away from his moping by the battle, and this intrusion of the political realm into his romance sets up one of the play's broad themes—the collision between personal interests and the needs of the state—that will return with a vengeance later on.
Meanwhile, the conversation between Pandarus and Cressida illustrates the way heroic conventions are undermined in Troilus and Cressida. In a standard heroic romance, Troilus would indeed be everything that Pandarus claims he is—noble, brave, handsome, courteous, and so on. But Shakespeare, by having Pandarus sing Troilus's praises, immediately calls into question the truth of those praises—is Troilus really great, we wonder, or is it just that Pandarus is trying to "sell" him to his niece? Meanwhile, the difficulties of structure in Troilus and Cressida are also evident here. As he does throughout the play, Shakespeare deliberately undercuts what should be a climactic scene: Troilus's appearance onstage with the other Trojan princes after the battle. Pandarus's praises, and the long delay before Troilus's arrival, make it seem as though the play is building toward a key moment—except that we have already seen Troilus, and so has Cressida, so there is nothing important about the scene!