Three months after Tom Mason agreed to go with the alien invaders, Weaver is still in command of the 2nd Mass, while Anne continues to serve as medic. Pope has taken command of the Berserkers, a renegade group within the 2nd Mass that Weaver doesn't fully trust.

Tom is still haunted by memories of his time with the aliens, while Pope plants seeds of doubt about him among the group. Ben's hatred for skitters grows and the 2nd Mass scouts locate a bridge that will help them cross the river to safety ahead of an impending attack.

The 2nd Mass moves into an airport hanger but the draughty conditions cause concern for the group's health. The Berserkers hatch a plan to get rid of Tom, and while dealing with a loss of one of its own, the 2nd Mass receives a promise of hope from a mysterious visitor.

While on patrol together, Ben and Hal try to help a young boy and wind up having their bikes stolen. The hunt to recover them leads them to the discovery of something much bigger. Also, Matt is put in danger, and Weaver is reunited with a family member.

A familiar face returns to the 2nd Mass, but his intentions remain unknown. Meanwhile, a skitter is captured and brought back to the camp as a prisoner, and Hal bonds with Maggie as she begins to open up about her past.

Hal stumbles across bodies of de-harnessed kids and finds someone from his past. Tom and Anne grow closer and Weaver's health deteriorates just as Tom discovers he may have been keeping important matters from the group in order to protect them.

Tom captures another hostage and Ben is placed in harm's way by someone who he thought he could trust. When members of the 2nd Mass are trapped in the basement of a building, the situation takes a turn for the worst when they find themselves faced with a new enemy.

The 2nd Mass pushes on to Charleston, but some are still grieving the group's recent losses. Weaver learns more about Tector's background before the invasion, and Matt begins to bond with a harnessed girl who was found recently.

The 2nd Mass meets new faces, while Tom comes across his former mentor and professor Arthur Manchester, but it's difficult to know whom to trust. Maggie begins to realise that Pope may not be sticking with the group very long, and Tector embraces his military past.

When skitters approach the newly combined resistance force, the groups' interaction surprises everyone. Meanwhile, Tom finds out life-changing news just as the 2nd Mass comes face-to-face with their most dangerous threat to date.