Thorin was the leader of the group of dwarves on their quest to the Lonely Mountain to reclaim the dwarves' home and treasure from the dragon Smaug. With him are his nephews Fíli and Kíli (sons of his sister Dís), his relatives Balin, Dwalin, Glóin and Óin, and the dwarves Dori, Nori, Ori, Bifur, Bofur and Bombur. They are joined by the hobbit Bilbo Baggins, and on parts of their journey by the Wizard Gandalf.
In the Battle of Five Armies his nephews Fíli and Kíli were killed while defending him, and Thorin himself was mortally wounded. He died soon after the battle, but lived long enough to reconcile with Bilbo. Thorin was succeeded as King under the Mountain by his second cousin Dáin II Ironfoot.