The entirety of Rogue One is actually summed up in two lines of the opening crawl of Star Wars: A New Hope, which reads: "Rebel spaceships, striking from a hidden base, have won their first victory against the evil Galactic Empire. During the battle, Rebel spies managed to steal secret plans to the Empire's ultimate weapon, the Death Star."
Now, Rogue One isn't exactly a sequel and it isn't exactly a prequel, though it's not as far removed as an anthology film might be. It isn't a full-on Episode, either, which is why it's been dubbed simply, "A Star Wars Story." And to set itself apart, it also ditches that iconic opening crawl and opts for a single title card: "A long time ago in a galaxy far, far away..."
That said, if you were expecting a crawl to set up the timeline of the movie and give you a quick "Previously on Star Wars," we've got you covered. Cue that legendary John Williams score and read on:
The GALACTIC EMPIRE has risen. Darth Sidious Palpatine has declared himself Emperor and, with the one-time Jedi Knight known as Darth Vader at his side, set about creating a planet-destroying weapon.
With Padmé Amidala dead, her twins hidden throughout the galaxy and the Jedi presumably extinct, the fledgling REBEL ALLIANCE is holding on to its last vestige of hope.
To crush that faith, the Empire has sent their director of Advanced Weapons Research to a remote farm planet to track down a polymath who has the final key to complete their Death Star...
If you need a refresher in even more layman's terms: Rogue One takes place sometime after Revenge of the Sith and before A New Hope. Anakin Skywalker-cum-Darth Vader (as played by Hayden Christensen, before he turned into James Earl Jones...somehow) was taken from a lava pit where he was left to die. Baby Luke and baby Leia have been split up and shipped off into hiding. Natalie Portman is dead. The Dark Side is winning. And, now you're set!
There's also this explanation for Rogue One: For decades, Star Wars fans have crying plot hole over the fact that Luke Skywalker just...shoots into a small hole in the Death Star and the whole things explodes to smithereens. Even if it's a "one in a million" shot, as Han Solo shouts, it's a bit insanely convenient. This is the two-hour movie to say Ah-ha! We already thought of that! Okay, NOW you're set!