|whoa... suddenly everything is all peaceful again at the start of season 3... well, except for the fact that all the spirits from the spirit world have moved to the human world and vines are growing everywhere and destroying Republic City...|
It doesn't explain what happened after the end of Book 2, but my theory is that Korra moved all of the spirits to the human world and closed the portals, so that Vatu's 10000 years of darkness would be limited to only the spirit world... but no that can't be right, he should have the ability to open the portals just like Korra does.
Also, Korra still has all of her bending abilities. It's clear that she can't contact the previous avatars anymore, but it looks like she can still use the "Avatar state" somehow. I'm so confused...
Anyway, ever since the harmonic convergence, more people have suddenly started to learn airbending. So now Tenzin's family and Korra aren't the only 5 airbenders left in the world... The president of Republic City is fed up with how much the avatar is screwing things up, and makes her leave the city. So now she's going on a journey with Boline and Tenzin's family to meet other airbenders. Tenzin convinced her that her actions are helping to restore balance to the world, and even if she makes some people hate her, it's still a good thing.
Also, this one guy who is in some prison in a desolated mountain area, gets airbending and uses it to escape. His name is Zahir... he says some vague, foreshadowing stuff that suggests he'll be the new major villain. It seems the people keeping him locked up were fire nation guards. Lord Zukko is told about this, and the fact that other villains are escaping now to... he warns that this group of villains is dangerously powerful..
Some kid named "Kye" who was about to be arrested for stealing from his orphanage, had just learned airbending and says he wants a 2nd chance, so he joins with Korra's group.. Genora starts flirting with him.
The avatar group gets to Ba-sing-se to look for more airbenders. The Earth Queen there is totally corrupt and is robbing the citizens with crazy taxes.. she hates the united Republic because they took some of the Earth kingdom's land away. Kye runs away from the group and starts using his airbending to steal from people.. Macco and Boline go to find him, and instead they find their long lost relatives and their grandma.. and they learn about how the Dye-lee agents have been kidnapping everyone with airbending powers and recruiting them for the Earth kingdom's army... Kye also ends up getting kidnapped.