At Calchas's tent, Diomedes calls to Cressida. Her father fetches her, while Troilus and Ulysses watch from one hiding place and Thersites from another. With Thersites's profanity and Troilus's shock providing a counterpoint, Diomedes woos Cressida, who behaves reluctantly but coyly toward his advances, fending him off for a time but never allowing him to leave. Eventually, she gives him a sleeve that Troilus presented to her as a love-token—then she takes it back, and says that she never wants to see Diomedes again—then she softens, gives it to him once more, and promises to wait for him later, when he will come to sleep with her. When she is gone, and Diomedes too, Troilus is in agony, first denying the evidence seen with his own eyes, and then pledging to find Diomedes on the field of battle and kill him. Finally, as morning nears, Aeneas arrives to lead him back to Troy.
In the city, Hector girds for battle, while the women—his wife Andromache and sister Cassandra—plead with him not to go. Both have had dreams that prophesy his death, but he dismisses their warnings. Troilus comes in and says that he will be fighting too; indeed, he chides Hector for having been too merciful to his enemies in the past, saying that today Troilus plans to slay as many men as he can. Cassandra leads Priam in, and the old king pleads with his son not to fight, saying that he too feels foreboding about this day, but Hector refuses to listen and goes out to the battlefield. Pandarus brings Troilus a letter from Cressida; Troilus tears it up and follows Hector out to the field.
As the battle rages, Thersites wanders the field, escaping death by brazen cowardice. The Greeks are being driven back, and Patroclus is killed; Agamemnon orders his body brought to Achilles, who is roused to fury and joins the battle. He duels with Hector briefly, but tires and retreats; Hector continues slaying, while Achilles finds the Myrmidons, his men, and sets out to find Hector again. Eventually, as the battle nears its close, Achilles and his men find Hector, who has finished fighting and taken off his helmet. Surrounding the unarmed Trojan, they stab him to death and then tie his body to a chariot and drag it around the walls of Troy. The Trojan soldiers are grief-stricken, and Troilus leads them into the city to bring the heavy news. On the way, he encounters Pandarus, and curses him. Left alone on the stage, the unhappy Pandarus wonders why he should be so abused, when his services were so eagerly desired only a little while before.
Cressida's "fall" to Diomedes bears a marked resemblance to her earlier "fall" to Troilus. In each case, there is a (pretended?) reluctance to go along with the wooer, and in each case she eventually yields. If one considers Troilus and Cressida a tragedy (an arguable notion) then this is the tragic climax: when Troilus finally realizes that his beloved is not all that he thought she was, and cries in despair,
This she? No, this is Diomedes' Cressida.
If beauty have a soul, this is not she;
If souls guide vows, if vows be sanctimonies,
If sanctimony be the gods delight,
If there be rule in unity itself,
This was not she.
His grief is genuine, even if one might point out that he should have known better, and his own good qualities are ruined as a result—in the final battle, he chides Hector for having "a vice of mercy" (V.iii.37), and abandons his old fealty to the idea of honor, giving in to bloodlust and the quest for revenge.
If Cressida's infidelity is the climax of the romantic plot, then the duel between Hector and Achilles should be the climax of the political narrative. We have the foreboding prophecies of Andromache, Cassandra, and Priam, and Hector's tragic refusal to listen to them, followed by the death of Patroclus, which finally rouses Achilles to action. The action all seems to be moving toward a great showdown. But as is true throughout the play, their confrontation is a letdown—instead of a climactic battle, we have a brief swordfight from which Achilles flees, and then the brutal murder of an unarmed Hector a few scenes later. This is a departure from the Iliad, in which Hector dies in a fair fight, but it is entirely true to Achilles's dishonorable character in Troilus and Cressida, and it brings the play to an end with an appropriate anticlimax—there is no revenge for Troilus and no justice for Hector, only sadness in Troy and the final speech by Pandarus the pimp.
Just before he is killed by Achilles, Hector fights a nameless Greek in armor. After he kills him, he remarks, "most putrefied core, so fair without, / thy goodly armor thus hath cost thy life." Since we are given no knowledge of the dead Greek's identity, it is an obscure passage, but the image of a beautiful veneer hiding corruption seems to speak for the entire play, in which noble warriors turn out to be brutes, and beautiful women are revealed as shallow and disloyal. It is, in the end, a profoundly pessimistic story—as Thersites says, "war and lechery confound all!" (II.iii.76-77).
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: