A man named Preacher (Martin Landau) walks into Mom's Diner on a snowy evening, sits at the counter, and orders "the usual". An unattractive fish on a platter is brought to him and Leo Bain (Donald Pleasence) the chef, comes out from the back and begins talking to him. It starts to rain right inside the diner. Preacher is hoisted up on chains and Bain swings a cane knife as Preacher sits up in his bed and screams. It was all a nightmare he is having while residing in a insane asylum.
The next day, Dr. Dan Potter (Dwight Schultz) arrives at the hospital to see the clinic director Dr. Leo Bain, and Bain hugs him even though they have never met. Bain is apparently just as crazy as most of the patients at the hospital. At home later, Dan is moving things into his new house with his wife Nell (Deborah Hedwall) and their 11-year-old daughter Lyla (Elizabeth Ward). That night at the hospital, one of the patients, Frank Hawkes (Jack Palance) gets out up of bed, places his hands near a window, causing a gate to shut and an alarm to go off. The next day, Dan arrives back at the hospital to meet with Bain again where Bain introduces Dan to his patients kept on the maximum security third floor, whom Bain calls "voyagers". They include Hawkes, a former World War II P.
O. W. who went insane and was committed for fighting and nearly killing a few recent Vietnam War P. O. W. s out of anger and frustration. The other is Byron Sutcliff, "Preacher", a former priest who was committed for setting fire to and burning down three churches believing himself to be a messenger from God.
Ronald "Fatty" Elster (Erland Van Lidth) is a 370-pound child molester and pedophile. John Skagg, "the Bleeder" (Phillip Clark) is a murderer who was committed for strangling two women and who gets nosebleeds whenever he kills. Skagg refuses to show his face before Dan. Dan tells the group that the previous doctor, Harry Merton, had to leave, but Hawkes expresses suspicion and hostility towards Dan. That night, Hawkes tells the others that he thinks Dan has killed Merton and believes that Dan intends to kill them.
Hawkes tells them that they must break out somehow and kill Dan. The next day, Ray Curtis (Brent Jennings) the lead orderly on the third floor, tells Dan that the four voyagers want to kill him for his stern rules and says that they are "intense". Ray pushes a button that shuts a steel door to his office and he tells the doctor that electricity is the only thing that separates him and the voyagers. Outside in the gated courtyard where the patients intermingle, Preacher asks Bain for a match and he uses it to set his suit coat on fire and swings it at people. Dan tries to stop him, but Preacher only pushes him aside in anger. Bain walks up to Preacher and whispers something in his ear which calms him down.
Bain tells Dan that he told Preacher that he threatened to hoist him up and "cut him in half" every time his misbehaves. In Dan's office, Ronald sneaks in and finds a letter address to Dan and a framed picture of Nell and Lyla. Meanwhile, Dan goes to speak with Hawkes about Merton, and Hawkes says he knows the doctor is somewhere in nearby Philadelphia. Dan arrives home that afternoon and he meets his visiting younger sister Toni (Lee Taylor-Allan) who arrives for a visit. Toni invites them all to a rock concert in town and during the drive, Nell lets Toni in about the patients Dan is treating and also lets Toni know why John Skagg is called the Bleeder. At a nightclub, Dan, Nell, and Toni arrive and watch a punk rock band named the Sick Fucks perform. Dan certainly does not enjoy the music.
Suddenly, the electricity for the place goes out. The crowd heads outside and sees that there is a blackout. In the hospital, Preacher sits up in his bed claiming "I made the lights go out!" Hawkes puts his hands near the window, and neither the gate nor alarm are active, so the four voyagers walk out of the room. Preacher and Ronald find Ray hiding in a closet in his office, and after Preacher punches and kicks him a number of times for his abuse, Ronald picks up Ray and breaks his back.
The group walks outside where the hulking Ronald punches through a car window to get at a doctor attempting to drive away and throws him out. The four psychopaths drive away in the doctor's car. At a nearby shopping center, the four voyagers arrive where a mob of looters have broken into several stores. Entering a sporting goods store packed with more looters, the four escaped maniacs don new clothes for themselves and arm themselves with a variety of weapons. Hawkes takes a crossbow with several arrows, plus a few handguns. Preacher takes a large Bowie knife and a double-bladed axe. Ronald takes only a baseball bat, and Skagg dons a hockey mask with a gardening fork.
Outside, Skagg "The Bleeder" swipes a man with the gardening fork, killing him, and apparently disturbing the other voyagers so much that they get into the dead guy's van and drive off, leaving the Bleeder behind. The next day, Dan arrives at the hospital to learn from Bain about the four escaped patients and of the people they killed. With the power still off from an apparent statewide power outage, Dan has to report to the police about the four maniacs on the loose. In a nearby neighborhood, Hawkes, Preacher, and Ronald are driving around and torment a bicycle messenger and when the man mouths off to them, calling Hawkes an "asshole" the enraged Hawkes throws the van in reverse and plows into the bicycle messenger. At Dan's house, Preacher arrives wearing the bicycle messenger's hat and uniform and tells Nell claiming that he has a telegram for the doctor. Nell offers to take it, but Preacher says that he will come back later. Nell is slightly spooked by Preacher's strange behavior.
After Preacher leaves, Toni tells Nell to accompany her to an anti-nuclear protest that is going on in town. A little later, after Toni and Nell leave, Lyla comes home to a vacant house, and Ronald is there, claiming to be her babysitter. Ronald is friendly towards Lyla, but she is a little unsettled by his strange behavior. Ronald suggests going to Lyla's room to show her what he can do with cut-out paper. Meanwhile, Toni and Nell are in jail having been arrested with dozens of other protesters. Toni meets a certain Tom Smith, one of those arrested with them during the demonstration. When it is time to make their phone calls, Tom lets Toni use his turn at the phone where she calls Dan at the hospital to tell them where they are and to bail them out. Dan then calls Bunky (Carol Levy), Lyla's real babysitter, to go over to his house to make sure that Lyla is okay.
Bunky arrives at Dan's house and after looking around, finds Lyla asleep on her bed. Bunky then calls her boyfriend, Billy, and invites him to come on over. Billy arrives a short while later and Bunky invites him up to the master bedroom so they can have sex. While making out on the bed, Bunky hears a noise coming from the closet and asks Billy to check it out.
He is pulled under the bed and the bowie knife is jabbed through the bed several times at Bunky who is able to dodge it. Bunky jumps off the bed and runs to the bedroom door. Just when Preacher comes out from under the bed, Bunky runs out of the bedroom only to be grabbed by Ronald who strangles her to death. Later, Dan arrives home with Nell, Toni and Tom when they see cops all over the house. Lyla is fine as she told them about Ronald who was there, but the police apparently do not know about the murdered Bunky and Billy. Tom asks to stay for dinner and Dan also invites Detective Barnett to stay as well in case any more of Dan's escaped patients pay a visit.
That evening, Dr. Bain is trying to get a hold of Dan on the phone, but he is getting no response. At nightfall, the Potter family is having dinner when Detective Barnett goes outside after hearing a noise. As the family and Tom watch from a window, a crossbow is fired and Barnett is hit and pinned to a tree. The group reacts quickly and begins making sure all the doors and windows of the house are locked, but they also learn that the phone is dead. Toni is too scared to go upstairs alone for she tells Dan that she once had a breakdown and spent time in a mental hospital as well. Tom goes upstairs instead and secures all the windows. When Dan looks back outside, he sees that Barnett's body is gone.
After an arrow is shot through a window, Tom and Dan barricade the front door with furniture as well as some of the windows as another few arrows are shot through the windows as well. Meanwhile, Bain is told by the telephone operator that the phone line to the Potter residence is out of order. He sets out to the Potter residence. At the house, the group hears a car approaching and it is Bain who gets out and tells them that their phones are out of order. Dan and the others yell at Bain to get back in his car and drive away. After Dan yells at Bain that the voyagers are there, Bain decides that it would be best to try to talk with them and calls them out. Preacher comes out of hiding from the nearby bushes and Bain (who is just as insane as all of the other psychos) tries to talk with him to come back to the hospital.
Preacher angrily responds by slashing off Bain's left year with his Bowie knife and then tries to get inside the front door when Dan opens it to try to help. Bain runs back to his car and Preacher turns back towards him and runs over beside the vehicle and yells "Romans 12. 19: Vengeance is mine, saith the Lord!" Preacher pulls out his axe and swings it. A minute later, Tom looks out of a window and no one is inside Bain's car. Dan tries to scream to the men outside and tells them that he did not murder Dr.
Merton, but he gets no reply. Suddenly, Barnett's dead body is thrown by Ronald through a window, and the group stacks furniture against it as Hawkes shoots another arrow through the broken window. Toni think she sees another window open and when she goes to close it, a bloody body jumps through the window at her. But she only imagined it.
When Dan asks her what is wrong, Toni says "I'm getting sick again. "Minutes later, when the group smells smoke, Dan realizes that Preacher must have broken into the basement and is planning to set fire to the house. When Nell goes to look for the fire extinguisher, she finds the dead Bunky and Billy in a closet. Dan runs down to the basement, and struggles with Preacher and knocks him out with the fire extinguisher, and puts out the fire. Upstairs, Ronald bursts into the house from the kitchen back door and runs for the group with his baseball bat. Lyla ducks under a table and cuts his leg with a knife, and Tom swings a large cleaver into his back. Tom then picks up Ronald's baseball bat and beats the cleaver in further, and Ronald falls over dead. Dan decides to make run for it and he runs outside to get to Bain's car to try to hotwire it to drive away and get help.
Inside, Tom embraces a shaken Toni.... and blood begins to pour from his nose. Nell sees this and immediately realizes who Tom really is. Tom/Skagg wraps his hands around Toni's neck and begins to strangle her. Dan runs back inside when he hears the screaming and grabs Tom away from his sister.
Lyla hands her mother the knife and Nell uses to stab Tom in the stomach, killing him. Suddenly, Preacher comes out of the basement and Dan struggles with him. Dan manages to twist the large bowie knife out of Preacher's hand, and stabs him the chest with it and throws the fatally wounded Preacher back into the basement. As Dan, Nell, Lyla and Toni gather together for comfort, Hawkes appears standing in the kitchen doorway with his crossbow aimed right at time. "It's not just us crazy ones who kill", says Hawkes. Suddenly, the electricity comes back on and Hawkes sees Dr.
Merton interviewed in a news report on television. Evidently upset, Hawkes breaks the TV and leaves the house and runs off into the night. Hawkes walks through the town and passes the nightclub where the Sick Fucks are playing. He enters the club and beats up the doorman outside who insults him. In the club, Hawkes watches the punk rock band perform, when a girl whacked out on drugs walks up to Hawkes and says something to him. He pulls out his. 45 caliber gun and points it at her neck. She looks at it and laughs, and so does Hawkes.