Back in the 1970s, several patients in an experimental sleep study reported seeing shadowy intruders. These patients - and hundreds of others - died in their sleep soon after. Doctors called the phenomenon 'Sudden Unexplained Nocturnal Death Syndrome'. They refused to discuss the shadows. Now, a struggling late-night radio host (Dallas Roberts) and a skeptical CDC investigator (Alison Eastwood) have begun researching a disturbing new outbreak of shadow sightings and sleep fatalities. Could this be a case of paranoid hallucination, or are these victims literally being scared to death by actual nightmare creatures?