A man known as Preacher (Martin Landau) enters Mom's Diner on a wintry evening, settling at the counter while ordering "the usual." What arrives is an unappealing fish on a platter, prompting the chef, Leo Bain (Donald Pleasence), to step out from the kitchen for a chat. Suddenly, rain begins to pour inside the diner, and Preacher finds himself bound by chains, facing Bain who threatens him with a cane knife. This chaotic moment jolts Preacher awake in his bed with a scream, revealing it all as a haunting nightmare within the gloomy confines of an insane asylum.
The following day, Dr. Dan Potter (Dwight Schultz) arrives at the hospital to meet with Dr. Leo Bain, who greets him with an unexpected hug despite their unfamiliarity. It quickly becomes apparent that Bain shares the same eccentricities as the hospital's patients. At home, Dan is seen unpacking alongside his wife, Nell (Deborah Hedwall), and their young daughter Lyla (Elizabeth Ward). As night falls, one patient, Frank Hawkes (Jack Palance), breaks free from his bed, jeopardizing the safety of the facility.
The next day, Dan returns to the hospital, where Bain introduces him to the more dangerous patients on the maximum-security third floor, referred to as "voyagers." Among them are Hawkes, a former WWII prisoner who was committed for his violent outbursts, and Byron Sutcliff, known as Preacher, who was institutionalized for setting fire to three churches, convinced he was a messenger of God. The group also includes Ronald "Fatty" Elster (Erland van Lidth), a large pedophile, and John Skagg, ominously dubbed "the Bleeder" (Phillip Clark), a murderer whose killings are marked by uncontrollable nosebleeds. Tensions rise as Hawkes sows doubt among the group, claiming Dan has sinister intentions towards them.
Later, head orderly Ray Curtis (Brent Jennings) informs Dan of the intense feelings the voyagers have toward him, hinting at their murderous designs. Outside, in the hospital courtyard, Preacher ignites his coat in an act of rebellion. Despite Dan's attempts to intervene, the situation escalates until Bain intervenes, whispering a calming message to Preacher.
That night, Dan is faced with increasingly suspicious behavior from Hawkes and his cohorts. After a series of chaotic events, including a blackout affecting the area, Preacher and his fellow patients seize the opportunity to escape, leading to a spree of violence on the streets. As they navigate a nearby shopping center, they adopt new identities, arming themselves with various weapons and leaving a trail of destruction behind them.
Back at the hospital, Dan learns of the escapees' atrocities and grows increasingly frantic as they impede upon normalcy. While Nell and her sister Toni (Lee Taylor-Allan) delve into their daily lives, they are suddenly haunted by Preacher's ominous reappearance at their home, bearing unsettling intentions and a sliver of deception. As darkness engulfs the evening, tensions mount in the Potter household, culminating in a horrifying confrontation when Dan returns home unexpectedly to a scene no one could ever anticipate.
As tumult reigns, Dan finds himself in a frenzied fight for survival, attempting to keep his family safe from the raving murderers hunting them down. The intensity reaches a peak as the once-ordinary suburban life becomes a battlefield, revealing the depths of madness that blur the lines between sanity and insanity. When nightmare becomes reality, the question looms: will any of them survive the unfathomable horrors awaiting them?
Their struggle transforms into a fight not just for survival, but for humanity itself, as they confront the chilling truth that Hawkes ominously concludes—> "It's not just us crazy ones who kill."
