Take one kid, kidnapped by aliens. He and his fellow captives break free, and he gets home with a small but sturdy spaceship.
Add one kid, whose father used him in experiments involving the transfer of memories from one person to another. The experiments never worked, and they stopped after his father was assassinated. Then he hit puberty, a time of life which causes all sorts of changes in the brain. One day, he woke up, and remembered that he was Rambo/MacGuyver/James Bond/Bruce Lee. Among others.
Add another kid, the world’s greatest computer hacker. Made himself rich by figuring out all the lost Bitcoin passwords. Uploaded an AI best friend into the internet, which gave him unlimited access to, well, everything.
All three independently discovered a next of aliens on their world. They all attacked it in their own way, and, working together, destroyed the nest.
Now they defend Earth from aliens – at first their Earth, now this new Earth.