Iphigenia in Aulis takes places just before the outbreak of the Trojan War. The Greek army is ready to make the long journey to Troy (also called Ilium), where they plan to attack the city in order to get back Helen, a mythically beautiful woman whom Trojan prince Paris has absconded with. Helen's Greek husband, King Menelaus, is upset by the loss of his wife, and his brother Agamemnon vows to help him get her back. Agamemnon raises an army of Greek warriors.
The Greeks' ambitions are foiled, however, by calm seas—with no wind, they can't set sail in order to get to Troy. Calchas, a seer able to read omens and interpret the will of the gods, tells Agamemnon that they must appease Artemis (the huntress goddess of virginity and maidenhood) by sacrificing his daughter, Iphigenia. Then and only then will they be able to sail for Troy and war.
Agamemnon reluctantly agrees, and tells his wife Clytemnestra that he has achieved a brilliant marriage for Iphigenia—Achilles, the greatest of all the assembled Greek heroes, wants to marry her before he sets sail. Clytemnestra and Iphigenia head to Aulis.
Agamemnon has second thoughts about murdering his daughter so he can go fight a war, but his brother Menelaus (distressed husband of Helen) intercepts his letter. Then it's Menelaus's turn to have second thoughts, but by this point Clytemnestra and Iphigenia have already arrived and Agamemnon, fearing what the assembled army will do if he refuses to sacrifice his daughter, declines his brother's offer to forget the whole war-to-get-his-wife-back thing.
Agamemnon tries and fails to get Clytemnestra to skip her daughter's "wedding." Clytemnestra runs into Achilles, who is bewildered by her references to his impending nuptials. The truth comes out, and Clytemnestra begs Achilles for help. Clytemnestra then goes to Agamemnon, and, together with her son Orestes, begs him not to kill her daughter. Achilles speaks to the army on Iphigenia's behalf, but meets with stony looks and stonier threats (they threaten to stone him if he prevents the sacrifice). He opposes Odysseus, who is in favor of her death and swift passage to Troy.
Iphigenia, however, cuts the debate short when she volunteers to be sacrificed for her country. Achilles, much moved, stations his men around her altar.
Here Iphigenia in Aulis ends, but the story doesn't. For more fighting, sacrifice, scandal, murder, and angry wives, check out the Oresteia by Aeschylus. (Also, all may not be over for Iphigenia—some stories say she makes it out just fine!)