Billions have died from the alíen intrusion of 2020. There is only a rough estimate of about 100,000 people left in the entire world. They all live in organized camps, but each city must have two fighters to kill off these aliens. Why only two? Because people give these aliens more power. If there are only two people in a one mile radius of them, the alíens have only a little bit of power.
They are still very deadly creatures to say the least. Two teenagers are chosen to fight the aliens, until they die and are replaced. After the teens are chosen, they must train and be released out of the gates. Then the horror begins. No food, No shelter, no safety, only weapons and aliens.