Once there was a great warrior called Achilles in the Roman version and Achilleus in the Greek. My spellchecker doesn't like the Greek version and that makes me really angry and so I will always write Achilleus just to annoy it and fans of the film because the film sucked.
Anyway, Achilleus was just walking along and killing the occasional Trojan when an old guy turned up at the Achaian ships (that's the Greeks to people who've only seen the film) and demanded that Agamemnon (hard to say isn't it?) return his daughter Chriseis who he had captured and enslaved while he was ransacking some random village that nobody cares about.
Now Agamemnon was the top dog and would not be humiliated in front of the other warriors so he chucked the old guy out on his ear. However the old guy was Chryses, a priest of the God Apollo and if there's one thing you have to remember in the late bronze age, it's don't piss off Apollo or any of his cronies.
So Chryses waved his staff a bit and shouted and eventually convinced Apollo to rain down poisonous arrows of plague on the Achaians (ewww!). After a while Agamemnon and the other great heroes the book seems to namedrop all the time, realised that their men were all dying of some horrible disfiguring disease and decided to check out what was causing it. They soon realised that Apollo was the culprit and that the only way to stop the plague was to return Chryses' daughter.
Agamemnon was pretty livid at this point and Achilleus' nagging didn't help any. The so called ruler of men decided that since he was going to be slavegirless that he would take Achilleus' slave girl out of spite. This is the point in the film where Briseis is captured, passed around a bit and made to be a heterosexual love interest and a lot more important that she actually is. Briseis is not a princess, priestess….whatever, she's a random object for sex, weaving and other household activities. Back in the day the loving relationships were between men and other men and not with women who were mainly servants and breeders.
Achilleus is mightily upset when Briseis is taken away, not because he loves her but because his pride has been damaged. So, he proceeds to run away and cry like a little girl. Then his mother, Thetis, who's a goddess don't ya know, comes out of the sea to see what she can see, ha ha. Anyway, she comforts him and Achilleus convinces her to ask Zeus to sway the battle in Troy's favour so that Agamemnon will realise that he is indispensable and beg for him to come back. Zeus agrees and Achilleus and his sort of cousin sort of lover Patroklos skip off back to their hut for wine and sex while chaos ensues on the battlefield. End of part one