The first season, Book One: Air, sees Korra move to Republic City to learn airbending from Tenzin, Avatar Aang's son. She enters the pro-bending league, and befriends the brothers Bolin and Mako, as well as Asami Sato, heiress to Future Industries, a leading engineering corporation. The ambitious politician Tarrlok enlists Korra to fight the anti-bender uprising of the "Equalists", led by the masked Amon, who strips benders of their abilities. Korra and her friends, aided by police chief Lin Beifong and United Forces General Iroh, unmask Amon as a bloodbender and Tarrlok's brother, ending the Equalists' coup. A spiritual meeting with her predecessor Aang allows Korra to realize her powers and to restore the bending abilities of Amon's victims.
The second season, Book Two: Spirits, begins six months later, with dark spirits terrorizing the seas. Korra turns to her spirit-attuned uncle Unalaq, chief of the Northern Water Tribe, for tutelage, and opens the polar portals to the Spirit World at his direction. Unalaq then seizes power in the Southern Water Tribe by force, starting a civil war in which he is opposed by his brother, Korra's father, Tonraq. Seeking allies against Unalaq, Korra experiences the life of the first Avatar, Wan, who fused his soul with the spirit Raava to imprison her opponent Vaatu. Aided by his twin children Eska and Desna, Unalaq frees Vaatu during the Harmonic Convergence, a decamillennial alignment of planets, and unites with him to become a dark Avatar. As Korra fights this figure, her link to the previous Avatars is broken; but with the help of Tenzin's daughter Jinora, she defeats Vaatu and Unalaq, and leaves the spirit portals open, allowing a new coexistence of spirits and humans.
The third season, Book Three: Change, begins two weeks later with people all over the world discovering airbending powers as a result of the Harmonic Convergence. As Tenzin, Korra, and her friends recruit them to re-establish the extinct Air Nomads, the newly airbending criminal Zaheer escapes his prison, frees his allies Ghazan, Ming-Hua, and P'Li, and attempts to kidnap the Avatar. This fails thanks to the help of Suyin Beifong, Lin's previously estranged sister. Zaheer and his team – members of the Red Lotus, an anarchic secret society – kill the Earth Queen, throwing her kingdom into chaos, and try to force Korra's surrender by taking the Air Nomads hostage, including Suyin's daughter Opal. In the final confrontation, Zaheer's comrades are killed and he is captured by the Air Nomads led by Jinora. Two weeks later, a weakened, wheelchair-bound Korra watches as Jinora is anointed an airbending master, and Tenzin rededicates the Air Nomads to service to the world.
The final season, Book Four: Balance, is set three years later. Korra slowly recovers from the injuries incurred in the fight with Zaheer, traveling the world alone and haunted by fear. Meanwhile, Suyin's former head of security, Kuvira, reunites the fractured Earth Kingdom with the metalbenders loyal to her, and refuses to release power to the unpopular heir to the throne, Prince Wu. At the head of her new, totalitarian "Earth Empire", Kuvira seizes Suyin's city of Zaofu and sets her sights on the United Republic, which she claims for her nation. With Toph Beifong's help, Korra frees herself of the remnants of Zaheer's poison, but after losing a duel to Kuvira at Zaofu, she allows Zaheer to help her overcome her fears and regain her power. When Kuvira attacks Republic City with a giant mecha carrying a spirit-powered superweapon, it takes all the efforts of Korra and her friends, including the inventor Varrick and his assistant Zhu Li, to stop the colossus. Only after Kuvira's weapon blows open a new portal to the Spirit World, destroying much of the city, does she concede defeat. The series ends with the prospect of democracy for the former Earth Kingdom, and with Korra and Asami leaving together for the Spirit World.