Set two weeks after the end of Book Two, Book Three finds Korra discovering that new Airbenders are popping up all over the Earth Kingdom. While she tries to find these Airbenders and rebuild the Air Nation, a group of dangerous criminals embark on a mission of their own.
After the events of Harmonic Convergence, Korra has her hands full with trying to remove all the spirit vines that have overgrown Republic City. Her lack of success strains her relationship with President Raiko and brings down her approval rate among the city's populace. When airbenders are suddenly popping up across the world and causing more trouble for the city, President Raiko demands Korra to leave the city, which she takes as an opportunity to travel the world to find the new airbenders. Meanwhile, a dangerous criminal uses his newfound airbending abilities to escape.
Tenzin, Jinora, and Team Avatar travel toward Ba Sing Se in order to find the new airbenders. Stopping at various villages along the way, they hope to recruit them in order to rebuild the Air Nation, though with little success as they were only able to convince a young boy named Kai to join them. Meanwhile, Zaheer travels the four nations to free his fellow criminals from their prison. After freeing Ghazan and Ming-Hua, they set course for the Northern Water Tribe to free their last member, P'Li. Upon learning of their escape, Lord Zuko sets course for the Northern Water Tribe himself in an attempt to stop them.
Team Avatar arrives in Ba Sing Se. While Korra has an audience with the Earth Queen, the others notice that Kai has gone missing, prompting Mako and Bolin to go look for him. They track him down, but he manages to shake them in the monorail, which takes the brothers to the Lower Ring, where they eventually run into their family from their father's side. Meanwhile, Korra and Asami set course for a small village to collect taxes as a favor to the Queen. After getting her money, the Queen brushes off Korra's request for help in tracking down the airbenders, leaving the Avatar to storm out of the palace. In the city, Kai found himself arrested by the Dai Li and subsequently conscripted in the Earth Queen's first airbending regiment.
Lord Zuko, Tonraq, Desna, and Eska try to prevent Zaheer from freeing the last prisoner, P'Li, while Korra discovers that the Earth Queen is forcing all the airbenders in Ba Sing Se to join her army. Team Avatar thus devises a plan to free them from the monarch's captivity.
Korra's quest leads her and her friends to the metal city of Zaofu.
Per Su's suggestion, Korra begins to learn metalbending. Opal admits her reluctance to leave Zaofu due to her obligations to her family while Bolin deals with his fear of learning how to bend metal. Meanwhile, Lin faces her past during an acupuncture sessions, forcing her to relive the circumstances which led to her and Su's strained relationship. After a cathartic release of her repressed emotions, Lin makes amends with her sister as well as Opal, who she convinces to travel to the Northern Air Temple to train with Tenzin. Elsewhere, Zaheer and his group determine Korra's whereabouts shortly after escaping Republic City, deducing that she was under the Metal Clan's protection in Zaofu.
Despite Bumi's bad attitude, Tenzin attempts to train the new members of the Air Nation with little success. Things become more complicated for the airbending master as Jinora, with Kai's influence, begins to exert her independence and demand her airbending tattoos. The new airbenders are tested when they are forced to face a group of bison rustlers who have taken Jinora and Kai along with several bison calves.
Zaheer and his gang try to kidnap Korra.
Korra learns the truth about the group that is planning to destroy her.
Korra and Asami are stranded in the desert. The members of The Red Lotus negotiate with the Earth Queen.
Mako and Bolin deliver an ominous message from Zaheer.
When other plans fail, Korra suggests a brave idea to take on the Red Lotus and save the Air Nation.