Pacific Rim came out back in 2013 and it hasn't been the sort of franchise to keep itself current by inundating us with media over the last six years. As such, it's very likely that even if you saw Pacific Rim back then, and even if you liked it, the story has probably faded from your memory beyond the basic set up of "giant robots vs giant monsters."
To be sure, "giant robots vs giant monsters" sums up Pacific Rim pretty well, and considering how few members of the first movie's cast will be returning, the sequel is likely to go off in its own direction. However, it will still be building on the events of the first movie, so here's a refresher course going into this weekend's Pacific Rim Uprising. Here are the key things to keep in mind.
The Kaiju War
The story of Pacific Rim starts in 2013, which is when, in this timeline, the first Kaiju appeared to wreak havoc on Earth. The planet banded together to create the Jaegers, massive robots used to fight and defeat the Kaiju. We then cut to seven years later to see the Kaiju adapt their attacks to overcome the Jaegers, killing Yancy, the brother of Raleigh Becket, the first film's lead character played by Charlie Hunnam. We then jump an additional five years into the future to watch humanity's last stand against the Kaiju. This put the end of the first movie in 2020, making the sequel take place sometime later.
Jaegers, from the German word for hunter -- as the movie makes sure to tell you -- are massive robots piloted by humans almost as a large exoskeleton. The pilots move their legs to walk the Jaegers and swing their arms to punch the Kaiju. Each Jaeger has a pair of pilots because it was discovered that a single pilot couldn't handle the mental load of piloting alone. The two pilots are mentally synced together via a process called drift, that allows them to work together seamlessly. However, this process also gives pilots a peek into the mind and the memories of each other. This means pilots have to be willing to get very close personally so compatibility is important.
Pan Pacific Defense Corps
The Pan Pacific Defense Corps is the global organization created to deal with the Kaiju threat. It's run in the first film by Marshal Stacker Pentecost, played by Idris Elba. The organization is very nearly scrapped entirely after the Kaiju threat becomes stronger, but when the alternate plan of building a massive sea wall proves to be useless, the PPDC is given one last chance to use the Jaegers to seal the rift once and for all. What happens to the organization by the end of the film is unclear, though trailers for Pacific Rim Uprising would seem to indicate the PPDC is going strong.
Where The Kaiju Come From
The Kaiju come through a rift in the Pacific Ocean sea floor. They're massive creatures that appear to exist only to destroy. During the events of Pacific Rim, the research division of the Pan Pacific Defense Corps, Dr. Newt Geiszler and Dr. Hermann Gottlieb, use the same drift technology as Jaeger pilots to interface with a Kaiju brain and discover that the Kaiju are artificial creations of an alien race that seems to be coming from an alternate dimension on the opposite end of the rift. The aliens travel from world to world stripping them of resources before moving on, and they want to make Earth their next home. They're responsible for the extinction of the dinosaurs, though Earth wasn't ready for their life form then, and now they're back.
We won't see Stacker Pentecost in Pacific Rim Uprising, but that doesn't mean the character won't be important. The former head of PPDC dies sacrificing himself so that Raleigh Becket and Pentecost's adopted daughter Mako Mori can make a run at closing the rift. He was going to die anyway as a result of radiation exposure as part of being an early Jaeger pilot. Mako Mori is one of the few characters that we do expect to see again in Uprising, but we also know that John Boyega will join the film as Jake Pentecost, son of Stacker. Stacker makes no reference to having a son in the first film, though he does tell Raleigh Becket at one point that Becket has "no idea" who Stacker is or where's come from, so there clearly is a lot to the character that we don't see in the first movie, part of that appears to be his estranged son.
The Rift Was Sealed
While many blockbuster movies make a point to leave a door open for any potential sequels, Pacific Rim did not do that. At the end of the first movie, the last working Jaeger, Gypsy Danger, goes nuclear in order to seal the rift between the dimensions. The rift collapses and the four Jaegers that were still functioning at the beginning of the final assault have all been destroyed. The story would seem to have reached a conclusion, but obviously, that's not the case. Somehow the Kaiju will return, and some all-new jaegers will be needed.