Rogue One is the first film in the Star Wars Anthology series, a collection of stand-alone stories set in the Star Wars universe. The story is set in between the events of Episode III: Revenge of the Sith (2005) and Episode IV: A New Hope (1977), and centers on a group of rebel fighters who come together to carry out the mission of stealing the plans for the Death Star.
Trivia from IMDB.Com
Feature film debut of bounty hunter Cad Bane, made famous from the Clone Wars television series.
It is rumored that Peter Cushing would reprise his role as Grand Moff Tarkin from Star Wars: Episode IV – A New Hope (1977), even though Cushing died in 1994. He would be resurrected via CGI and archive footage from his films.
Hayden Christensen could possibly appear as Darth Vader and James Earl Jones voicing Darth Vader. This film is set after Revenge of The Sith also before A New Hope, so Darth Vader will appear due to the story of the film.