The Astromech droid is in service to the Empire and is used to maintain ships and other equipment. But unlike R2-D2, this unit isn't granted independence and is apparently subject to regular memory wipes. As for how this little unit fits into the story, we will have to wait and see.
Set between Star Wars: Episode III - Revenge Of The Sith (2005) and Star Wars: Episode IV - A New Hope (1977), Rogue One: A Star Wars Story follows a group of Rebel soldiers who set out on a dangerous mission to steal the plans for the original Death Star and bring hope to the galaxy.
Our hero this time is Felicity Jones' (The Theory Of Everything) Jyn Erso, a conflicted fighter who is a rebel in more ways than one. She leads up a cast that also includes Diego Luna (Milk), Riz Ahmed (Ill Manors), Ben Mendelsohn (Killing Them Softly), Donnie Yen (Ip Man), Jiang Wen (Gone With The Bullets), Forest Whitaker (Lee Daniels' The Butler), Mads Mikkelsen (The Hunt), Alan Tudyk (Serenity), Jonathan Aris (Sherlock), Eunice Olumide (World War Z), and Genevieve O'Reilly reprising her role from Star Wars: Episode III - Revenge Of The Sith (2005) as young Mon Mothma.
Rogue One: A Star Wars Story is released worldwide on 16 December.