In the aftermath of an alien invasion that devastates the globe, a colony of survivors come together to share supplies and stay alive. Tom Mason goes from history professor to soldier, appointed the second in command of the 2nd Mass.

Tom, Hal, and their squad are taken hostage by a gang of marauders lying in wait for them at a weapons supply depot, and become bargaining chips in the hands of their leader, John Pope.

A colleague of Tom's late wife joins the 2nd Mass with a viable plan to remove the alien harnesses from abducted kids -- but the two men have personal issues to work out. Meanwhile, the plan to rescue Ben is derailed when a member of Tom's team finds his own son first.

Tom allows Pope to join his team on a mission to retrieve motorcycles, and encounter a disturbing new enemy army. Anne and Dr. Harris disagree over how to deal with their Skitter prisoner, and Mike tries to reconnect with his son.

Hal plans a quiet operation to extract Ben from behind enemy lines under the cover of night, but must first find a way to kill a Skitter without making any noise. Captain Weaver orders the Skitter prisoner executed, and Anne struggles over the memory of lost loved ones.

The leader of the 7th Mass shows up with a warning of an impending Skitter attack, and controversial orders to send the camp's young people ahead to safety.

The 2nd Mass braces for another attack, unaware that Lt. Clayton has ulterior motives for the children at his safe haven. Pope escapes his captor, and Anne and Maggie prepare for the birth of Sarah's baby.

Weaver struggles with his personal demons during a reconnaissance mission, during which he, Tom, and Hal find a woman living alone in enemy territory. Pope is tasked to construct a weapon, and Anne makes a disturbing discovery during an alien autopsy.

With increasing doubts cast on Weaver's ability to lead the 2nd Mass, Tom confronts him in the hours leading up to a major assault on an alien structure -- resulting in a rift inside the settlement. Ben learns more about his connection to the enemy while helping out Scott in the radio room.

As the resistance moves to strike against the Skitters, Rick flees the camp in order to warn the enemy -- forcing Tom to evacuate the civilians from the school. Weaver embarks on what is sure to be a suicide mission, but Ben helps develop something that may give them a strategic advantage.