It's another tale of the Empire versus the Rebellion. Except that the Empire is now called the "First Order" and the Rebellion is now called the "Resistance". For some reason. Anyway, it's pretty clear that just killing off the Emperor 34 years ago was no magic elixir for restoring freedom to the far, far away galaxy.
As we left things, plucky young Rey has tracked down Luke, the Last Jedi, demanding that he return with her to help out against the Dark Side of the Force. He's gotten cranky in his old age, and demurs, but Rey hangs around.
Meanwhile, the ragtag Resistance fleet (they should really use Ω for their symbol) is being pursued by the massive First Order fleet, which is methodically picking off ship after ship; the good-guy population is in serious decline. So the other plot thread involves the increasingly futile efforts of Princess Leia and her retinue (including last-movie heroes Finn and Poe) to stave off destruction long enough to reach safety.
Without spoilers: The overall tone is grim, although there are occasional flashes of humor. Many of the heroic good-guy efforts turn out to be either (a) just delaying the inevitable, or (b) pointlessly misguided. I'm not sure how I feel about that.
It's also very very long.