The entirety of the plot isn't known in great detail however a rough outline can be gleaned based on what we know so far. That is, specifically:
- Luke Skywalker hasn't been seen in some 20-30 odd years, and that the Skellig Island shoot containing a very old Monastery is where Luke has been hermit-izing himself all these years. There was a board in which the cast and crew had access cards for flight by chopper to the site each day, and the cast portion only contained to boxes, one for Mark Hamill (it was only MH, and images showing luke inside the monastery confirm his character's presence) and a solitary "K", which is considered to be "Kira", Daisy Ridley's character. Reports also confirm she was flown in and out of the site a few times as well, so we can confirm that she goes to confront him. Sources also state this scene is roughly one minute, and contains a large significance for the overall film.
- The beginning of the film is rumored to be that of Luke's severed hand or lightsaber from Episode V falling down to a desert planet, where Daisy Ridley finds it, and takes it to (we assume) Andy Serkis' character, or Max von Sydow's character. That person is a historian and collector of sorts, and recognizes it for what it is, and eventually sets the above paragraph in motion, except when John Boyega's character is a part of an Imperial attack on the town that the collector and Daisy visited or live in. Boyega for some reason unknown, doesn't obey orders, and eventually hooks up with Daisy and they wind up in contact with Oscar Isaac's character (the X-Wing Pilot), who was shot down during the attack (rumored to have been, anyway - this is the newest info). They tow his X-Wing using some kind of pig creature (set pictures out there, showing it), and eventually hook up with the Millenium Falcon.
Let me know if you want me to keep going...