King Ahasuerus of Persia is having a party, and he wants to show off his wife, Queen Vashti, but she refuses to come perform for him. So he sends her away and has a contest where all the beautiful young virgins in his kingdom are brought to him one by one, and whichever one pleases him the most gets to be the new queen. Esther, the daughter of a Jewish man named Mordecai, wins this contest. Mordecai later warns her of a plot to kill the king, and she passes along the information, saving the king's life.
The king then gets a new favorite advisor named Haman. Mordecai doesn't treat Haman with reverence, so Haman gets angry and schedules, for a particular day, a massacre of all the Jews in the kingdom. Esther decides to intervene for her father, and convinces king Ahasuerus to eat with her so that they can talk, but for some reason tells him that she'll discuss her concerns with him the following day. Haman is present for this meal, which he thinks is a sign of favor from Esther. He decides that night to hang Mordecai and builds a gallows for him (seemingly not realizing that he is the queen's father).
King Ahasuerus then is reminded of Mordecai's assistance in saving his life, during a bout of insomnia. He asks Haman what he should do to honor a great man, and Haman says (thinking that the great man is himself) that the man should get to parade around town dressed up as the king. Ahasuerus likes this idea and forces Haman to do this for Mordecai.
Esther has dinner the following night with Ahasuerus and Haman, and tells the king that Haman plans to slaughter her people. The king storms out in anger, and Haman gets up onto the bed Esther was lying on, in order to beg her to spare him. The king walks in and thinks that Haman intends to rape the queen, so he has Haman hanged on the gallows Haman had built for Mordecai. The king then allows Mordecai to author a new edict that changes the day of slaughter against the Jews to a day of slaughter by the Jews against their enemies. This commandment causes many people to convert to Judaism out of fear.
On the appointed day, the Jews kill many people, including the sons of Haman, whose bodies are then hung up on trees. This day then becomes a Jewish holiday, and everyone lives happily ever after (with Mordecai as the new favorite advisor to the king).