What old wrong do the dwarves wish to right?

Many years earlier, the wicked dragon Smaug drove the dwarves away from their kingdom under the Lonely Mountain and their ancient treasure. An important part of the treasure is the Arkenstone of Thrain, a great jewel that is precious to the dwarves. Thorin Oakenshield is the grandson of the last dwarven king, and he persuades Bilbo Baggins to help them reclaim the treasure and the throne.