In the first scene of the Iliad, Chryses, the priest of Apollo, comes to ask for his daughter back, whom Agamemnon has been keeping as a slave.
Even though the other Achaians tell him to give her back, Agamemnon tells the priest to get bent.
Then the priest calls in a favor from Apollo, who sends a plague upon the Achaians. After nine days of plague, Achilleus calls a meeting of the Achaian chieftains.
There, Achilleus backs up the right to speak of Kalchas, the seer. Kalchas correctly diagnoses the source of the problem and instructs Agamemnon to give the girl back.
Agamemnon curses out Kalchas and then says he'll only give the girl back if he gets something equal in return from one of the other chieftains.
When Achilleus opposes this, they get in a heated argument which results in Agamemnon demanding to take Briseis, Achilleus's girlfriend.
As we all know, this makes Achilleus super-mad. He almost kills Agamemnon, but then winds up just quitting the Achaian team. Oh yeah, and getting Zeus to help the Trojans destroy them.
Then Agamemnon sends Chryseis (the daughter of Chryses) back home.
That night, Zeus sends a dream to Agamemnon. In the dream, Zeus tells Agamemnon that Troy will fall if the Achaians attack immediately, in full force.
The next morning, Agamemnon relays this message to his generals. Then, for reasons that aren't exactly clear, he says they should first test the soldiers' loyalty by telling them they can go home.
As you might expect (but as Agamemnon apparently didn't), as soon as the soldiers hear this they all start racing for their ships. Only through the interventions of Athene and Odysseus do things get back under control.
Then Agamemnon puts his soldiers into order by city of origin (as per Nestor's advice) and they march off to battle.
When he sees Hektor calling a truce, though, Agamemnon stops his soldiers from shooting at him. This allows the two sides to negotiate the one-on-one duel between Paris and Menelaos.
Obviously that doesn't work out. Then, when Menelaos gets shot by the Trojan archer Pandaros, Agamemnon starts freaking out, and Menelaos has to calm him down.
Once he's pulled himself together, Agamemnon becomes an effective motivator of his soldiers – even if he offends Odysseus by questioning his courage.
At one point, once the battle is joined, Agamemnon sees his brother about to spare some surrendering Trojan. Agamemnon says, "Less sparing, more spearing." Which he does.
Later on, when Hektor proposes another one-on-one match, Agamemnon prevents Menelaos from accepting the challenge. Then when Nestor calls all the Achaians cowards for not accepting the challenge, Agamemnon is among those who volunteer to draw lots. Of course, the lot falls to Aias, not Agamemnon.
As it turns out, Hektor and Aias's duel is inconclusive. The next day, Agamemnon consents to Priam's request for a truce to bury their dead.
Two days later, when the Trojans have driven the Achaians inside their new wall and camped on the field, Agamemnon makes a defeatist speech to the Achaian leaders, suggesting they sail home.
But when Nestor suggests they ask Achilleus for help, Agamemnon immediately agrees. He says he was not in his right mind when he insulted Achilleus that way. He also offers Achilleus an extremely generous consolation present – including an offer of marriage to his own daughter. In addition, he promises to swear an oath that he never slept with Briseis.
Of course, Achilleus rejects this offer.
Understandably, that night, Agamemnon can't sleep. He goes outside and sees his brother, Menelaos, who also can't sleep. They assemble a bunch of chieftains who eventually decide to send a spy out into no-man's-land. This develops into Odysseus and Diomedes's night raid.
The next morning, Agamemnon distinguishes himself in battle by killing various Trojans.
Eventually, though, he gets stabbed in the forearm by a warrior named Koon. Obeying a signal from Apollo, Hektor now counterattacks. In the meantime, Agamemnon goes back to nurse his wound by the ships with some other wounded leaders.
A little while later, when Nestor brings Agamemnon the news about how badly the Achaians are doing, Agamemnon once again suggests they head for home. This time, Odysseus argues against him, with some deep insights into the psychology of teamwork.
Agamemnon consents to the advice of Diomedes – that, even though they are wounded, they should go out and encourage the men.
By the time the next day rolls around, Patroklos is dead and Achilleus has received his new armor. In front of all the Achaians, Achilleus formally swears that he isn't mad at Agamemnon any more.
Agamemnon accepts Achilleus's apology, and swears that he didn't mean to insult him; he explains this using the temporary insanity defense.
Agamemnon then offers Achilleus all the same gifts he did before. Even though Achilleus doesn't seem to care too much, Odysseus goes and gets the stuff. When he brings it back, Agamemnon takes his oath about not sleeping with Briseis.
After Achilleus kills Hektor and buries Patroklos, Agamemnon attends the funeral games. When the final competition – spear-throwing – rolls around, Agamemnon steps up as a contestant.
Achilleus decides to award the prize to Agamemnon without even holding the contest. He says he knows that Agamemnon is the best.
Agamemnon, in turn, gives his prize to Talthybios the herald.