Falling Skies begins six months after a global invasion by extraterrestrials, where in only days, the invaders neutralized the world's power grid and technology, defeated and largely destroyed all the world's militaries, and killed over 90% of the human population by destroying all of the world's major cities and capitals. The aliens include mechanical attack drones called "Mechs"; a species of light brown-skinned six-legged beings known as "Skitters" that appear to control the mechs; and a mysterious species known as the Overlords, or "Espheni", presumably the actual engineers of the invasion and the masters of the Skitters.The aliens' objectives are not explained until season 4. They plan to extract helium-3 from Earth's moon to power their technology, and to use humanity as an enslaved front-line army in their war with another alien race. To do this, the aliens round up children between the ages of 8 and 18 and attach a bio-mechanical mind control harness to their spines. Forcibly removing it generally kills the child, but midway through season 1, a surgical method is developed that allows a harness to be safely removed, leaving in place the "spikes" that connected the harness to the spinal column.
https://www.youtube.com/watch?v=CILkO-NCL8c
