With Haman's plan to kill Mordecai and all the other Jews almost fully foiled (they just need to officially reverse his orders), everything else starts to look up for the heroes too.
King Ahasuerus gives control of Haman's house to Esther, who then gives it to Mordecai. Mordecai also gets the signet ring (bearing the royal seal) that Haman had had formerly.
Next, Esther weeps at the king's feet and begs him not to go through with Haman's plan (which you would tend to think was already pretty much over, considering the king just had Haman killed).
The king agrees, authorizing Mordecai to revoke the decree using the king's seal.
Mordecai has letters written in all the different languages of the king's empire, telling people not to go through with the massacre and giving the Jews permission to defend themselves using violence, if necessary.
Mordecai is decked out in royal robes and a crown and the people of Susa rejoice. All the Jews are joyful. They celebrate in their different towns and cities when the good news arrives.
Even people who aren't Jewish claim to be Jewish, because they're afraid of reprisals now that Haman's plot has been overturned.