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 biomechanical 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. In Season 5, it is revealed through an Espheni communication device -- which Ben can interact with by touching it, due to having the spikes -- that there is a being superior to the Overlords, known as "the Queen." In the series finale, the Queen explains that the invasion about which the entire series is based upon is the result of a prior Espheni attempt to invade Earth, the only habitable planet in this galaxy and thus of immeasurable strategic importance. Her beloved daughter led that invasion, but the Espheni underestimated humanity and the invasion was stopped. The Queen's daughter was killed and the Queen swore to wipe out humanity in revenge.
The story follows a group of survivors who band together to fight back. They call themselves the Second Massachusetts, an allusion to the historical regiment from the Continental Army. The group is led by retired United States Army Captain Dan Weaver. Boston University history professor Tom Mason is second in command and must put his extensive knowledge of military history into practice while searching for his son Ben
Under the Dome tells the story of the residents of the fictional small town of Chester's Mill where a massive, transparent, seemingly indestructible dome suddenly cuts them off from the rest of the world. With no Internet access, no mobile signals and limited radio communication, the people trapped inside must find their own ways to survive with diminishing resources and rising tensions. While military forces, the government, and the media positioned outside of this surrounding barrier attempt to break it down, a small group of people inside attempt to figure out what the dome is, where it came from, and when (and if) it will go away.