In the Greek camp, the newly arrived Cressida is greeted by all the Greek commanders. Ulysses insists that she be kissed by everyone, only then refusing to kiss her himself—and when she is gone, he declares that she is a loose, unvirtuous woman. Then the Trojan lords arrive, and the conditions of the duel are set by Aeneas, who remarks that since Ajax and Hector are related, Hector's whole heart will not be in this fight. As the two combatants prepare, Agamemnon asks Ulysses "what Trojan is that same that looks so heavy?" (IV.v.94-95). Ulysses tells his general that the downcast Trojan is Troilus, and then goes on to praise him profusely, saying that Troilus may even be a greater man than Hector.
Ajax and Hector fight for a time and then break off, agreeing to call the duel a draw and embrace as kinsmen. Then Hector is invited to come unarmed to the Greek tents, since Achilles desires to see him; Hector agrees to come, accompanied by Troilus. He meets the Greek commanders, and greets them one by one, exchanging compliments down the line until he reaches Achilles, with whom he trades insults. Achilles promises to meet him on the field of battle the following day, and kill him; Hector retorts that he looks forward to their meeting. On that note, the Greek lords lead their guests to the feast. As they go, Troilus asks Ulysses where Calchas's tent lies, planning to find Cressida there later that night. Ulysses promises to lead him there, but also notes that Diomedes has been looking at Cressida lustfully.
After the feast, Achilles boasts to Patroclus of how he will kill Hector the next day. The two encounter Thersites, who delivers a letter to Achilles, and then unloads his usual torrent of abuse on them and on the entire campaign. The letter is from the Trojan princess whom Achilles loves, and it begs him not to fight the next day; he tells Patroclus sadly that he must obey her wishes. They go out, and Thersites remains; he watches from the shadows as the feast breaks up. Most of the lords go to bed, but Diomedes slips off to see Cressida, and Ulysses and Troilus follow him. Noting that Diomedes is an untrustworthy, lustful rogue, Thersites follows him as well.
When we last saw Cressida, she was in tears over being separated from Troilus, and promising to be true to him forever. Her transformation, however, proceeds quickly—she is cheerful and even coquettish upon her arrival in the Greek camp, adapting to her new situation with ease and bantering with her captors. Ulysses, ever wise, sees her for what she is—he suggests that everyone kiss her, and when she accepts their kisses without complaint, he declares that "her wanton spirits look out / At every joint and motive of her body" (IV.v.56-57). The reader may protest that he is somewhat unfair to Cressida, since the entire kissing routine was Ulysses's idea in the first place—but at this point, Shakespeare ceases to be fair. Cressida has been treated with sympathy up till now—or at least, she is given as much sympathy as anyone in the play receives—but now she must be degraded into the wanton woman that the audiences of Shakespeare's time expected from the already-known Greek story.
Meanwhile, the play's habit of anticlimax continues in the duel between Ajax and Hector, toward which all the action seemed to be leading, which falls flat. The two fight for a moment, grow weary, and then call a truce; clearly, neither one meant to kill the other. We are meant to be more concerned with the things going on around this supposedly central event, especially the exchange of insults between Hector and Achilles (foreshadowing the final duel—although that will also be an anticlimax) and the keen observations of Ulysses. Achilles sinks even lower in our estimation, since he cannot even manage to be courteous to a guest and is absurdly boastful to Patroclus (his "masculine whore," in Thersites's words) about how he will kill Hector the next day. Meanwhile, Ulysses's skills as a spymaster are again on display, since he picks out Diomedes's intent to pursue Cressida immediately and leads Troilus to her tent to watch—although whether Ulysses is actually doing Troilus a favor by showing him his beloved's infidelity is open to debate.
The last words before Troilus's illusions about his beloved are dashed belong, appropriately, to Thersites: "Nothing but lechery!" he cries. "All incontinent varlets," and one feels that his disgust is justified.
I just finished reading Troilus and Cressida in my effort to read all Shakespeare by his 450th birthday. It wasn't a favorite play, and I probably could have had a very happy life without ever reading it. But in case you're interested, here's my take: