Shaw learns the Republic is embroiled in a civil war with the Tricians, a ruthless faction which has taken control of and enslaved many caverns and cities to form their own nation. With his knowledge from the surface, Shaw joins a team of Republic agents: an escaped Trician slave, a brilliant scientist, a beautiful government agent, and a steam-powered robot, all seeking the means to recreate an artificial star in the underground. Success will mean limitless light and energy for the citizens of the Republic, while failure means oppression and slavery.