Television reporter Angela Vidal and her cameraman, Scott, are on assignment following the night shift of a Los Angeles fire station for a reality TV series. When an emergency call dispatches the crew to an apartment building, Angela and Scott enthusiastically cover the action, documenting the firefighters' efforts to save residents from what initially appears to be a routine distress call. However, upon their arrival, they encounter police officers already on scene responding to bloodcurdling screams from one of the apartments.

The building is quickly sealed off by authorities, trapping Angela, Scott, the firefighters, residents, and law enforcement inside. They soon discover that the cause of the chaos is much more terrifying than they could imagine—a deadly and aggressive strain of an unknown virus is spreading among the tenants. With the virus turning infected individuals into hostile beings, the group must navigate the dark corridors and confined spaces of the locked-down building, fighting for survival while searching for a way to escape the horror that is closing in on all sides.