Teacake and Naomi, two young employees of a self-storage company built on the site of an old US military base, have their wildest night shift ever when a parasitic fungus escapes from the lowest sublevel of the base, where it was sealed by the government decades before. As the temperature rises underground, this highly contagious microorganism multiplies and unleashes its terrors on the facility's inhabitants - human and otherwise. With time running out, it's down to Teacake and Naomi, with the help of a grizzled bioterror operative, to contain the rapidly mutating menace and prevent the explosive extinction of Mankind itself.