500 feet beneath the ocean's surface, the U.S. ballistic missile submarine Colorado receives orders to fire its nuclear arsenal at Pakistan. Captain Marcus Chaplin (Andre Braugher) and XO Sam Kendal (Scott Speedman) question the orders and the Colorado comes under fire from another U.S. submarine. With their submarine crippled, the crew of the Colorado are forced to seek refuge on a nearby island. They must survive unprecedented odds as they work to unravel the conspiracy and clear their names.

The series tells the story of the aftermath of a global invasion by several races of extraterrestrials that neutralizes the world's power grid and technology, quickly destroys the combined militaries of all the world's countries, and apparently kills over 90% of the human population within a few days. It's a chaotic aftermath of an alien attack that has left most of the world completely incapacitated. In the six months since the initial invasion, the few survivors have banded together outside major cities to begin the difficult task of fighting back. Each day is a test of survival as citizen soldiers work to protect the people in their care while also engaging in an insurgency campaign against the occupying alien force.