Rey feels Finn's distress elsewhere in the galaxy and says she needs to go to him. Luke tells her about the time he felt his friends in distress and Yoda told him to stay and finish his training, and his disobedience led to pain. But then he says he isn't Yoda, and has his own way of doing things, and tags along with her to go help.
Finn wakes up on a Resistance cruiser and is told by Poe that they're already en route to the makeshift Republic homeworld in the wake of the capital planet being blown up by Starkiller Base.
Leia wants to lobby them to go to war.
The Republic is being led by an interim dictator named Lord Vikram (Benicio Del Toro), who was quickly put into power by the few senators still around to maintain order.
Starkiller's destruction of the Republic core worlds caused a galactic communications disruption because everything was routed through those worlds, so Vikram is trying to reestablish contact and ensure the Republic doesn't collapse into anarchy.
Leia and Vikram butt heads. Vikram agrees that the Republic has to go to war with the First Order, but wants to wait for communications to come back online so they can muster up a sizable force (all they have right now is a single small fleet). Leia disagrees, and wants to take the war to the First Order now while Snoke is still reeling from Starkiller's destruction. She says both sides are in chaos, which makes this an opportune time to strike. Vikram counters by telling her she's letting her personal emotions over Han's death get in the way, and Leia throws it back at him by telling him he was always a little snake of a politician when they would argue back in her senate days, etc, etc, they have old bad blood or whatever.
The Republic planet is safe because it's protected by a heavy duty shield that can repel any invasion or bombardment from the First Order.
Finn and Poe are tasked by Leia to investigate the city because one of Leia's contacts informed her that the First Order have spies embedded in the Republic. Leia suspects Vikram.
Finn quickly becomes enamored by Leia's contact, Chala (Kelly Marie Tran), who tags along with him and Poe through the underbelly of the city to gather information.
Finn and Chala have a flirty romance subplot that involves a nighttime adventure through the rustic old city (this is the second entry in the trilogy after all, and poetry demands it).
Finn, Poe and Chala all corner the culprit: Vikram's aide, who has evidence of contact between Vikram and the First Order. Leia and some Resistance fighters confront Vikram and throw him in jail for treason. He denies everything, and the rest of the Republic officials are horrified because Leia basically just coup-de-etat'd the **** out of their leadership. Leia assumes control of the planet's defenses and absentmindedly puts Finn in charge of the shield codes.
Finn and Chala have a moment and Finn mentions his confusion about why Vikram didn't just lower the shields for the First Order. They walk into a room and Phasma is standing there and Chala draws a gun on Finn and forcibly takes the codes for disarming the shield from him. She says she planted the evidence, used Finn to get the codes, and basically that she thinks he's a dirty traitor to the First Order. In a moment of pure pottery, Phasma makes Finn lower the shields.
The First Order fleet jumps out of hyperspace and invades the planet, quickly capturing Finn, Poe and Leia. Kylo starts to torture Finn using the Force, knowing that it will draw Rey out of hiding and into his trap.