Fortunately, that night the king has insomnia. He asks his servants to read to him from his annals and they end up reading him the story of how Mordecai saved him from the assassination plot.
The king remembers how this was actually a pretty nice thing to do. He decides to honor Mordecai, who basically didn't receive any honors for saving the king's life up until then—a state of affairs the king wants to remedy.
The next day, Haman comes into the court, intending to recommend hanging Mordecai on the mega-gallows Haman has just built.
But before he can even get a word out, the king asks him what should be done for the person he (the king) wants to honor.
Haman thinks the king is talking about him, so he recommends a whole bunch of nice things: he should be dressed in royal robes and led around town on one of the king's own horses, wearing a royal crown on its (the horse's) head. He'll be led through the town in a procession announcing his honor.
The king says that this should be done for… (wait for it) Mordecai. Haman doesn't say anything back. He just carries out the order, leading Mordecai around the city on a royal horse.
Haman is naturally pretty upset about this. He mopes about it to his wife Zeresh and to his friends. They tell him that his plot to kill the Jews probably won't prevail if Mordecai is being honored like this.
As the chapter ends, Haman departs for the banquet, soon to discover his fate.