Life during star wartime
Since there's no expositional title crawl at the beginning of the movie, a little setup is needed: Rogue One starts in the years before the 1977 movie that launched the series, with the Galactic Empire holding a tight grip over the universe. To rein in portions of the galaxy that resist rule, the Emperor has commissioned the construction of the ultimate weapon: the Death Star. But the genius inventor behind the planet destroyer, a man named Galen Erso (Hannibal's Mads Mikkelsen), is in hiding. While it doesn't take long for Imperial douchenozzle Orson Krennic (Bloodline's Ben Mendelsohn) to haul him back to the diabolical drawing board, Galen's departure allows for his young daughter, Jyn, to go into hiding.
The thrust of Rogue One tracks grown-up Jyn (Theory of Everything's Felicity Jones), now a Han Solo-y ruffian making do on the fringe, as she falls into cahoots with the Rebellion. The anti-Empire insurgents, led by unlikely assassin Cassian Andor (Y Tu Mamá También's Diego Luna) want to track down and kill Galen Erso before he can complete the Death Star project. Jyn wants to find her father to understand what the hell is up. They band together to infiltrate Imperial bases, track down the imprisoned scientist, and eventually, steal a set of Death Star blueprints that reveal the 2-meter-wide thermal exhaust port just begging for a pair of proton torpedoes.