Falling Skies tells the story of the aftermath of a global invasion by extraterrestrials. Within a few days the invaders neutralized the world's power grid and technology, destroyed the militaries of all the world's countries, and killed over 90% of the human population by destroying all of the world's major cities and capitals. The aliens' objectives are not explained, though the aliens try to round up children between the ages of 8 and 18 and attach a biomechanical obedience device onto (and into) their spines, referred to as a ""harness"". The harness controls the child's mind, and forcibly removing it generally kills the child.
The story begins six months after the invasion and follows a group of survivors who band together to fight back. The group, known as the Second Massachusetts, is led by retired U.S. Army Captain Dan Weaver and Boston University history professor Tom Mason who, while in search of his son Ben, must put his extensive knowledge of military history into practice as one of the leaders of the resistance movement.