I've heard many rumors. most of them contradictory.
from what I can gather the plot goes something like this:
(this is ALL rumor and speculation -- not "spoliers" per se -- but "spoiler warning anyways")
- Luke is in exile. and/or, luke is being held captive by an Inquisitor, either way, nobody's seen him in 30 years.
- Daisy Ridley is Han's daughter. she is force-sensitive. she's upset that Luke never trained her.
- she's looking for her uncle.
- there's a half-man / half cyborg dude on tattooine. apparently he escaped from a sarlaac at some point. he used to be a Very Famous bounty hunter. he used to wear a helmet. with a range finder. until he fell into a sarlaac. now he collects artifacts from the OT (including, but not limited to, Master Replicas lightsabers).
- Daisy ridly seeks him out, because he's in possession of Luke's ESB lightsaber. and severed hand. (eww..)
(meanwhile)
- Boyega is a stormtrooper. he learns of a new secret weapon, similar to the death star, he steals a TIE fighter and flees the empire.
- he crashes the TIE fighter on tatooine and hooks up with Daisy Ridley.
- together these 3 escape Tattooine in the millenium falcon, piloted by Han Solo, who comes to rescue his daughter and take her off-world.
- they are pursued (?escorted?) by a squadron of Xwings.
- one of the Xwing pilots is Han's son. also force-sensitive.
- they find luke.
- luke has become too powerful. he levitates objects in his sleep. he can't control the force; the force controls HIM. he channels BOTH sides of the force. this makes him insane. he tries to contain it, by going into exile, to protect the galaxy.
- when Our Heroes finally find Luke, he takes one look at Han's son and he sees a vision of the future -- that Han's son will become a sith lord.
- he tries to kill Han's son.
- Han gets in the way --> luke kills Han.
- this causes Han's son to become angry, and seals his fate -- now he WILL fall to the dark side;
- Han's daughter becomes the next new hope, for the light side.
- now there are 2 vessels to channel both sides of the force (han's son vs. han's daughter) -- Luke snaps out of it --> the Force Awakens.
- Luke swears to train the daughter; the son swears revenge against Luke; the battle of Good and Evil continues for another generation.