This article is an excerpt from our Sefer
Buy me on Amazon.com
Bigsan and Seresh: 
Bigsan and Seresh, two servants of the king, plotted an assassination attempt against him [to poison him]. This plot was discovered by Mordechai which had Esther inform the king in his name. The two vandals were hung.
What were the jobs of Bigsan and Seresh?
They were in charge of supplying king with drinking water at all times. The Midrash states that Bigsan and Seresh were the personal guards of the king and they were later demoted from their duty in favor of Mordechai.
Why did they plot to kill Achashveirosh?
From the day that he married Esther he was constantly with her and hence became increasingly thirsty. Due to this they were always on call to supply him water and never got any sleep. [Thus it was Esther that indirectly caused Mordechai to save the kings life and hence set him up for his later glory. This later led to the downfall of Haman, who himself led to the appointment of Esther as queen. Thus ironically, we see throughout the Purim story how Haman was the source of his own downfall in many different aspects.] The Midrash however states that Bigsan and Seresh were upset because they were demoted from being the king’s guards and were exchanged for Mordechai. Thus they wanted to kill the king and hence show that while they were guards the king was protected, in contrast to the guarding Mordechai.
How did they plan on killing the king?
They planned to place poison inside the drinking water. The Midrash states that they entered a poisonous snake into the waters and then removed it.
Why did Mordechai reveal the plot to Achashveirosh; Didn’t he want Esther freed?
He wanted to gain favor in the king’s eyes in order to be given permission to rebuild the Temple. Alternatively he did not want people to say that while Achashveirosh was married to a Jew, suddenly he died. Alternatively he did not want people to say that while Achashveirosh was guarded by a Jew he died.
How were Bigsan and Seresh proven guilty?
Although they removed the snake from the water Hashem did a miracle and created another snake in the barrel and the snake was found.
Why was Mordechai never rewarded for his actions until much later on?
Achashveirosh mistakenly thought that Haman was the one who saved his life against the assassination attempt, as the scribes of the king were the sons of Haman and they lied and wrote that it was Haman who saved the kings life. Thus when the night came that the king’s sleep was disturbed the angel Gavriel read the correct version and this infuriated Achashveirosh that the true savior was never rewarded.