There is a gap of several years between TLJ and the new one. Four years because that's how long it was between the release of TFA, which TLJ took place right after, and the time the ninth will come out. In that time Leia died, RIP Carrie Fisher, but she managed to get allies and resources to rebuild the Resistance. The First Order has taken over the the inner sections of the Galaxy, but the Colonies and the Outer Rim are either independent or with the Resistance. Rey has found other force users and trained them in the force a little, creating a sort of Knights of Ren, but with the Resitance. Speaking of the Knights of Ren, they have returned and are now led directly by Kylo Ren and act as his enforcers to hunt down and kill or capture Rey and her force wielders. The war is different than Rebels versus Empire. While the Resistance does use smaller ships, they have the resources to make enough of them that a fleet of Resistance can reliably take on a First Order fleet. Poe is a fleet general, promoted in the power vacuum that arose in TLJ, while Finn is a spymaster, training and directing the spies that infiltrate the first order, using his stormtrooper training to do so. He also acts as a general when the need arises. That is what happened between movies. All this exposition will come in either the opening title crawl or in the first act of the movie. This could come by showing characters doing their jobs, a fleet battle showing Resistance tactics, and/or just spread out in sublte ways. The main plot of the movie is about the Resistance needing to fight the war while worrying about the fact that individual planets are considering leaving the Resistance and banding together to fight a defensive battle, rather than trying to get rid of the FO. While this is happening, Rey and her students and allies are trying to find a way to get rid of the Knights of Ren and stop the dark side. They've set up base on an uncharted planet rich with kyber crystals and containing an old Jedi temple with a couple holocrons that are used to teach Rey and the others more advanced, but forgotten, Jedi techniques. The Knights of Ren have set up base in the old Jedi temple on Coruscant, but Ren has completely remodeled it into a fortress and base for the Knights. I have just completely lost myself. If anyone wants to finish the movie using all that as a set up, please do so. I dont have enough imagination to come up with a good plot using all that.