The Rebel Alliance sends criminal Jyn Erso with an intelligence officer, Cassian Andor, to retrieve an Imperial defector who’s being held by an extremist named Saw Gerrera. Not coincidentally, Saw raise Jyn after her father, Galen Erso, was forced to return to Imperial service as a weapons designer and her mother was killed. The Alliance also wants to find Galen, who turns out to be one of the prominent figures behind the design of the Death Star.
Brief opinion: Far better than The Force Awakens, and that is entirely due to the last third of the film.
(Update, 18 Dec 2016: I watched the movie a second time, so am adding any extra impressions to this post. Text will be in blue.)