When the food and water supply was running low and people were starving and screaming for help, the government decided to cut down the amount of people. They took the old, the weak, and malformed newborns, and "sacrificed" them on spot, but that wasn't enough. The people were having kids and the population was growing again, so they started the Choosing.
The Choosing is a cruel method of population control where everybody must be taken from their parent and tested once they reach the age of 18. Then, the subjects go through a series of inhumane tests to see if they are "worthy" of living.
When Terra is snatched from her family by the claws of the government, she does something nobody's ever done before.
She fights back.