This is one fantastic game. I'm having a ton of fun with it. The instructions are not well explained though. I only now know pretty much all there is to know about the basic game system.
Each of your ten humans per level will need to be rescued, one at a time. When you hear the woman say "Keepers Detected", that's when green flying ships will come to kill them. If you don't find and kill all of those green ships before they get away, your human will die in his cell.
Once you kill them all, a shot will fly towards a human cell - follow it because that human is free walking around until something kills him. Follow the green arrow that then appears on your ship to know if you should fly left or right to get there faster. Once you touch them for the rescue (you can only carry one at a time), carry him to one of the two floating rescue stations by touching anywhere mid scree or above once you reach it (look for the arrows under it to know the full transfer area). You can "throw" a human towards the range area under the rescue stations, and if you Boost+Throw, the human will fly farther.
Sometimes an enemy will abduct a human and fly them straight up off the playfield like the old Defender/Stargate games. You have to shoot the ship before it gets away to save the human, who falls to the ground for you to pick up. Save all ten humans in a level for a huge bonus after the boss is defeated.
EDIT AndyN, I'll look into that tip, thanks. I'm on the last level on Normal difficulty. The game is not very long, but has a lot of replay value.