My take.
01- The Empire isn't really defeated in Episode 6. The Sith were. That remnant became the First Order and are still ruling the majority of the Galaxy but are now seen like "good" guys since it's not corrupted... for now. There's even force user amongst their rank (that shining silver Stormtrooper) while the rest of the Alliance still see them like bad guys and are still doing skirmishes in a resistance fashion.
02- The key source is the lightsaber Vader lost in Episode 6.
03- A race living on Jakku (I still think it's Tataouine) recovers it and turns it on.
04- For some reason I haven't really thought on, Kylo Ren senses it (or tracks it) and sends the First Order to capture it.
05- Jakku battle rages on.
06- Just like the Force sended Gui-Jon on Tataouine to discover Anakin, we find out that Rey is the daughter of Han and Leia, not in hiding, but there because it's not the place of a child to be at war. So she basically grew up without knowing. Leia tries to contact Luke and also send his husband. Rey was left with relative, BB-8 and unknown to her the Falcon.
07- We then discover that the First Order is still corrupted when Kylo and that shinning Stormtrooper general orders total annihilation until the artifact is recovered and his recovered by Finn's unit.
08- Finn is obviously not a killer, dislike what he thought was good and sabotage his unit and flees with the artifact.
09- Finn ultimately encounters Rey and they both try to runaway from the battle.
10- Some timely helps from Poe's squad, which includes Han and Chewie, gives them some respite. Han is reunited with his daughter and now the cast goes back to recover the Falcon which lie hidden.
11- After some battle, they recover the famous Falcon and run away with it. The shining Stromtrooper, as a Force sensitive, lays a trap and we see what we see in the teaser.
12- They ultimately succeed in running away and go back to the resistance hideout where everyone his reunited. They give the artifact to Queen Leia and wonder why it's so important to the First Order.
13- Rey finally understand who she is when Luke tells her.
14- Everything is cut short when Kylo Ren "discover"/track the base.
15- Battle and battle and ultimately the Resistance needs to flee. Leia will be killed in that battle by the shining trooper in a shocking twist...
16- When we discover Kylo Ren is a clone of Luke gone wrong (which is why the mask and the weird behavior) that escaped an abandoned cloning facility several years ago.
17- He needs Darth Vader lightsaber to open it again to heal himself and create the perfect, in his twisted mind, jedi order.
18- Ends.
It's basically taking a bit from Zahn trilogy.