In the first part of the two-hour series premiere, the world is left in tatters after a sudden, unprovoked and unexplained invasion by aliens, leaving the planet's remaining human population to fight for survival against the occupiers. History professor Tom Mason (Wyle) has been named second in command for the 2nd Mass, but his real focus is on his three sons, one of whom has been captured by the aliens for an unknown purpose.

While scouting for supplies, Tom, Hal (Roy) and their squad encounter a gang of marauders determined to get their hands on the 2nd Massachusetts' weapons in the second part of the two-hour series premiere. Leading the gang is an ex-con named John Pope (Cunningham), who decides to use his hostages as bargaining material.

Dr. Michael Harris (guest star STEVEN WEBER -- Happy Town) joins the 2nd Mass with a theory about what might save the teenagers abducted by the aliens. But the plan could be derailed when Tom starts digging for answers to questions that have haunted him since the early days of the invasion. The 2nd Mass comes face-to-face with skitters and mechs when they attempt to rescue Ben.

Tom and his team head out to track down more motorcycles for the 2nd Mass. -- relying on Pope to guide them to a good location. They hit the jackpot until they meet some skitters and harnessed teenagers at the bike shop. Back at the base, Dr. Harris and Anne disagree over the best method to gather more intelligence on the aliens.

Hal comes up with a dangerous new plan to rescue the captured teenagers from the aliens, but it might require putting someone at risk. Meanwhile, a sudden tragedy causes Anne to make an enormous discovery that proves instrumental to Hal's risky plan.

Lt. Terry Clayton (guest star HENRY CZERNY -- The Tudors) of the 7th Mass arrives with word that the aliens are on their way to attack the 2nd Mass. Clayton's plan to lead the children to a safe haven while the adults remain behind to fight is met with some resistance from the parents, including Tom.