"The best intentions invite the worst kind of trouble." - Balthier
I don't want to give anything away but here is the basic storyline.
Final Fantasy XII takes place in Dalmasca. Princess Ashe of Dalmasca and Prince Rasler of Nabradia were getting married, but the marriage was interrupted because Archadian Empire invaded Dalmasca. A war started, and Archadia took over Dalmasca. While the war was going on, Prince Rasler died. The king of Dalmasca was going to write down that he would listen to Archadia, but Archadia's ruler wanted him to be killed. Some knights from Dalmasca tried to rescue him, but one of the leaders of the knights, Basch, killed two other knights named Raminas and Reks. Reks was only 17 years old. Marquis Ondore told everyone in Dalmasca that Princess Ashe killed herself because she was so sad.
Two years later, Vaan, Reks' little brother, sneaks into the royal palace. When he was sneaking in, there was a dinner party going on to to celebrate the election of a new Consul, Vayne. Vaan meets the sky pirates Balthier and Fran in the palace, who teach him about magicite. When Resistance forces broke into the palace, Vaan and his new friends had to hide in the sewers. While they were in the sewers, they met the leader of the Resistance, named Amalia. That led to Vaan, Balthier and Fran getting put into the Nalbina Dungeons.
In the dungeons, Vaan, Balthier and Fran find Basch.