In The Desolation of Smaug, the hobbit Bilbo Baggins, and thirteen dwarves, while traveling to Lonely Mountain, pass through the black forest of Mirkwood, where they are attacked and captured by giant spiders. The spiders are then attacked by the Wood Elves, led by Legolas and Tauriel. Tauriel in particular saves Kíli by killing a spider that was attacking him, but the Elves subsequently take the dwarves as prisoners for trespassing on their land. During the dwarves' imprisonment, Tauriel forms a romantic bond with Kíli. It is later mentioned that Tauriel is a talented warrior and was therefore made leader of the Mirkwood border guards. Legolas, the son of Mirkwood's Elven king Thranduil, is indicated to be attracted to her, and her to him, but as she is a lowly Silvan Elf, she does not believe herself worthy of him. Thranduil, in fact, makes it clear that he does not consider her to be a suitable match for his son and warns her not to give Legolas false hope.
When the dwarves escape with Bilbo's help, the pursuing Elves are attacked by orcs, during which Tauriel again uses her fighting skills to save Kíli, though Kíli is struck in the leg with an orc's arrow, which Tauriel later learns from a captured orc is a Morgul-cursed weapon and will slowly kill Kíli. Just before Thranduil seals off his kingdom upon learning that an evil entity has returned and is amassing great power in the south, Tauriel goes after the dwarves by herself. Legolas follows, and initially tries to convince her to return, but when Tauriel refuses, saying that the Elves are part of Middle Earth and that the impending war must eventually involve them, Legolas joins her pursuit.
After acquiring provisions at Esgaroth, the dwarves leave for the Lonely Mountain, but leave Kíli, Fili, Bofur and Oin behind due to Kili's injury. By the time Tauriel and Legolas reach Esgaroth, the dwarves are being attacked by orcs. They repel the invaders, and Tauriel uses her knowledge of herbs and elvish medicine to heal Kíli's wound, saving his life once again. Upon recognizing his feelings for her, Tauriel appears greatly touched and seems to be developing feelings for him as well.