Something has oozed out from the vents atop the roof of the multi-story high rise Cooper Center Hospital, foaming and rolling, and when helicopter pilot Bobbi attempts to land upon that very roof with her paramedic and physician assistant they could tell as soon as they landed that the only way to escape was to enter the building and find their ways down.... mere steps ahead of an ever-spreading poisonous mat of rapidly-evolving purple fungus. Floor by floor as they descend, they encounter horrors which defy logic and fight to survive....