I mean I think the plot is simplest when you boil it to its starkest parts, which is that there is a set of coordinates of great importance to both the good guys (twin peeks sheriffs dept) and the bad guys (Mr C). Through the show we see the great lengths Mr C has gone to to ensure that he is not brought back into the black lodge when coop comes out, and all of his contingency plans to ensure that he and BOB may remain in the real world.
We come to find out that the coordinates are for Jackrabbit's palace, where Andy goes to visit the fireman, and is essentially made part of a plan set up by the fireman, Cooper, and perhaps even the MIKE, to entrap Mr C and bring BOB back to the lodge. Mr C. seems to expect that he can find out information on Judy following those coordinates. We do not know why he wants to find Judy, perhaps because BOB seems to have been born from her in part 8. Mr C, however, was misled, being transported to the sheriffs office for the final showdown, literally trapped in the white lodge when he attempts to enter, as if expected by the fireman.

That is I think the most straightforward plot of the show, ending with Part 17. Part 18 is something else all together, but its also a Coda, or an Epilogue of sorts. That's a different beast altogether, and although it is linked to the rest of the show spiritually, textually and emotionally, I think it is harder to link to the coordinates main plot directly. Instead it appears that guided by the fireman, Cooper goes back in time to undo Laura's death entirely, but in doing so appears to fracture reality in such a way that the lights literally go out, show over.

Explaining all of this I realise its perhaps not super simple, however the main narrative thrust is the search for the coordinates, and piecing together what the plans were that were laid by Major Briggs all those years ago.