Director Zack Snyder has talked with IESB.Net a little more about his prequel/sequel to "300" and revealed a little of the storyline.
The action will take place between the Battle of Thermopylae and the Battle of Plataea, the battle that narrator Dilios is at while re-telling the story of King Leonidas and the mighty Spartans to a new group of warriors at the end of the first film.
Frank Miller is writing and drawing the graphic novel that the second film will be based on. Upon completion of the book they will start writing a script for the movie based on it.
Snyder adds he won't have any input into the work as he wants it to be a creation entirely of Miller's.