FIRST ACT:
Rey, a scavenger with dreams of leaving the planet, finds the damaged Millennium Falcon within the husk of a crashed ISD. She immediately starts to repair it.
Finn is a stormtrooper in the first order. He witnesses an atrocity by Ren, and learns two things: that the Jedi are real and alive and that the pseudo-empire has a Mega Death Star. He deserts on a Tie Fighter, gets shot down and crashes on Jakku.
Finn and Rey meet up. Finn convinces her to help him find the Jedi. Together they set out to escape the planet while being hunted by Captain Phasma. They hustle to the Millennium Falcon.
Han and Chewie, years after being seperated from the Falcon due to some plot device return to meet Finn and Rey on board. Together, they make their escape from Jakku and set course for Forest Planet where the rebellion and Luke are at.
First Order recognize the Falcon and follow it.
SECOND ACT:
Falcon lands at Forest Planet. Han, Chewie and Finn set out to find Luke. Han, Chewie and Finn are attacked and apprehended by the First Order. Resistance X-Wings intervene. During the fight Chewie dies to save Han. Big fight over Forest Planet, First order retreats.
After the destruction, Han is heart broken and lost. Leia tries to console Han, with whom they have been seperated for quite a while now.. Rey receives Luke's lightsaber from Leia, who reveals she is her daughter from Han they left on Jakku to stay safe from the fighting the way Luke was left with Beru and Owen. (Plot twist, the future Jedi is NOT Finn, but Rey)
Luke is revealed. Having felt that there is 'balance in the force' following Vader and Emperor's death, he has taken up a hermit's life. Learning about the Knights of Ren forces him to act, he unites with his niece and trains her. Rebel fleet quickly follows and evacuates.
THIRD ACT:
Mega Death Star (AKA Frost Planet) blows up Forest Planet.
Rebels attack Frost Planet. They send a team on the ground to disable (plot device) so that the X-Wings can do the Trench Run. Ground team consists of Rey, Finn and some other people. Knights of Ren kick their ass, in the fight Kylo disables Rey and Finn has to pick up her lightsaber in a desperate last stand. Luke intervenes and dies in the process, enabling Finn and Rey to escape.
With (plot device) not deactivated the Trench Run fails. Poe is shot down and captured. Resistance fleet disengages and retreats. Kylo force-interrogates Poe into revealing the fleet's location.