When Zack returns to Moon the following summer, things are not going well for the fledgling lunar colony. The crew is beset on all sides by the remnants of Machvel's forces, hidden amidst their own ranks. Even the early, egalitarian political system, the First Democratic Order of Space (F-DOS), shows signs of cracking and splitting the crew in two. It is hard to tell friend from foe as Zack and his friends struggle to help Professor Goodspeed maintain peace.

Thankfully, Zack, Angelina, and Sanjay get a break from the constant conflict by taking in the new Mech Gladiator battles in the recently complete Mech Battle Dome. Zack befriends the best Mech Gladiator in the league and learns to hone his own skills in Mech battle.

However, troubling news soon reaches Zack and his friends that Machvel is mounting a massive attack, and this time, the odds are distinctly against the colonists of the Citadel Spire...