When King Agamemnon kills a deer in the sacred grove of Artemis, the goddess keeps the Greek fleet from sailing off to Troy. The only way Agamemnon can get his massive fleet sailing off to war is to sacrifice his daughter, Iphigenia, to the goddess of the hunt. As you might imagine, Agamemnon's wife Clytemnestra is pretty ticked off that he killed their daughter. So, she does what any reasonable person would do: when he returns home victorious from the war, she kills him with an ax in the bathtub.
Agamemnon, the king of Mycenae, is the most powerful dude in Greece. (Everybody's afraid of this guy.)
When his brother Menelaus' wife, Helen, runs off with Paris of Troy, the brothers round up all the bigwig Greek kings to go teach the Trojans a lesson.
Together, the two draft a bunch of famous dudes, like Odysseus, Ajax, and Achilles to go put a hurtin' on Troy.
Preparations for the war are going well; they've put together the biggest army of Greeks like ever, and most of the men are chomping at the bit to go defend Greece's honor.
The army hits a major bump in the road, though, when Agamemnon makes the mistake of shooting a deer in a sacred grove of Artemis, goddess of the hunt.
In some versions, it's said that he brags that he's a better hunter than her too.
To punish Agamemnon, Artemis causes storms to ground the Greek fleet in the place called Aulis.
In other versions, the goddess makes it so there's no wind at all. Either way, the Greeks are stuck.
A seer named Calchas tells Agamemnon that the only way Artemis is going to allow the fleet to sail to Troy is if he sacrifices his daughter, Iphigenia.
(Fun fact: back in the day, some cults of Artemis sacrificed young virgins all the time. Okay, maybe that fact isn't so fun.)
The big guy is not too happy about this, but he feels like he has to do it anyway.
Agamemnon summons his wife Clytemnestra, telling her to bring Iphigenia to Aulis.
(Sometimes the detail is included that Agamemnon tricks his wife and daughter into coming by saying that Iphigenia is to marry Achilles.)
Clytemnestra is furious when she finds out why Agamemnon wants Iphigenia there. (Uh, yeah. We can see why.)
Despite his wife's desperate protests, though, Agamemnon takes Iphigenia and sacrifices her to the goddess.
(In a couple famous variations, like Euripides' Iphigenia at Aulis, the girl is saved at the last second by Artemis, who whisks the girl away and replaces her with a deer on the altar.)
Whatever the case, the sacrifice makes Artemis chill out, and she finally allows the Greeks to set sail to Troy.
The horribly bloody Trojan War goes on for years and years.
(For the best account of Agamemnon's shenanigans during the war check out Homer's Iliad.)
Clytemnestra has plenty of time to stew while Agamemnon is off fighting the war.
For some weird reason, she just doesn't think that a war to get her floozy sister, Helen, back to Greece is worth the life of her precious daughter.
To get Agamemnon back, Clytemnestra starts sleeping with his cousin, Aegisthus.
Oh, and she also plots to murder her husband whenever he gets home.
Eventually, Agamemnon arrives home to Mycenae after totally annihilating the city of Troy.
The triumphant King brings home Cassandra, a Trojan princess, who he's forced to be his own personal sex slave.
(Man, he's really making this easy for Clytemnestra.)
Cassandra is a seer who can totally see into the future, but was cursed by Apollo to never be believed.
So, when she tells Agamemnon that they're both going to die in Mycenae, he's like, "Whatever."
Cassandra's prophecy totally comes true, though.
Clytemnestra acts all nice to her husband when he gets there and even makes him a nice warm bath.
Just as he starts to close his war-weary eyes she throws a purple robe over him, so he can't get up, then starts chopping on him with an ax.
(Sometimes, Aegisthus is said to help with the murder as well.)
Clytemnestra also kills Cassandra, who saw it coming, but could do nothing about it.
So, Clytemnestra and Aegisthus are now in charge of Mycenae.
However, Clytemnestra and Agamemnon's kids, Electra and Orestes, are going to have a little something to say about their father's murder in a few years' time.