According to George Lucas at Celebration III, in the series, "a lot of the issues from the films are connected, but you won't necessarily see a lot of the people that are connected." McCallum says on the writing and plot of the series: "We are just starting to interview writers and trying to really figure out which direction to go to. He envisions somewhere like 100 hours between Episode III and Episode IV with a lot of characters that we haven't met that have been developed in some of the novels and other things. We are really excited about that. Finally, we could have the opportunity to answer everybody's questions once and for all by the time we finish the series." Regarding the series, he says: "It is going to be much darker, grittier. It's much more character-based." The show is still in its early planning stages, so many plot and/or story elements may change, as Lucas has been shown to change his mind easily, such as expressing an interest in expanding the character Aayla Secura, then killing her in Episode III.
In a recent interview, when asked about the show, McCallum replied, "Think about bounty hunter, that’s all I can tell you."
In Star Wars Episode III: Revenge of the Sith, Palpatine, Supreme Chancellor of the Galactic Republic, declared himself Emperor and transformed the Republic into the Galactic Empire. In the process, he ordered the execution of Order 66, in which all members of the clone army of the Republic turned on their Jedi leaders and killed them. In a speech he delivered to the Senate, however, he reported that there were remaining Jedi, and that these survivors would be "hunted down and defeated".
The series is set between 19 BBY and 0 BBY, referred to as "the dark times" by Obi-Wan Kenobi in A New Hope. It is during this period that the newly formed Galactic Empire will rise to ultimate power throughout the galaxy. It is during this time period that the first Death Star will be constructed, albeit with many problems along the way.
However, most of the stories will be told from the perspective of minor characters from the Star Wars galaxy. From George Lucas' own words, we may see the notorious bounty hunter Boba Fett play a significant role in the series, with Daniel Logan reprising his role from Attack of the Clones. According to Lucas, several of the major characters from the films may have small appearances as well.
Over the course of the show, the fledgling Rebel Alliance will gradually form into a full resistance against the Empire. The Alliance, with its loyalist intentions to restore the Old Republic, will ultimately win its first battle against the Empire, setting up the events for A New Hope. Per the opening crawl of A New Hope, only one major Rebel victory can occur in the series. However, it is possible that the Rebels may achieve minor victories or even suffer defeats before that.
Recent Star Wars productions have revealed all that is known at this point about the period the series will take place in:
The Evasive Action: Prey webstrips reveal that an unknown number of Jedi escape Order 66 for any number of reasons (not actively involved in the war, 'conscientious objectors', semi-rogue, skill, luck, etc.), and are hunted for a time by Darth Vader. At some point, Vader is told to focus on helping Palpatine build the Empire, and less on remaining Jedi, leaving that to others.
Yoda is in self-imposed exile on Dagobah, while Obi-Wan becomes a hermit on Tatooine after taking baby Luke to live with Owen and Beru Lars. The Last of the Jedi: The Desperate Mission reveals that Obi-Wan resides there until contacted about Anakin's one time rival, Ferus Olin. He then goes on one final adventure off-planet before returning to Tatooine. He may have some further planetbound adventures, but otherwise mostly watches over Luke, from a distance.
A.C. Crispin has stated that she proposed a book series dealing with Princess Leia Organa between Episode III and IV, but that "Lucasfilm didn't approve the idea of a Leia backstory because they want to keep that era of the SW continuity untouched for the television series they're considering."