Harlan Thompson, David McCallum, is the head of the biology department at Southwestern University, a small college perched in an isolated southern California town. He dresses casually, mingles with students, and consistently clashes with fellow professors and administrators, whom he views as shallow elitists more concerned with status than true education. Yet he shows a soft spot for a young, newly hired biology professor, George Wyner Michael Fitzgerald, and tries to shield him from the corrosive campus system. The only other SU employee he seems to trust, at least a little, is his ex-girlfriend Caroline Donoghue, Sandra McCabe, though their relationship has cooled since their breakup.
Lately, a troubling pattern emerges in the countryside around the town. Several cows owned by local rancher Larry Ludecky have been found dead. Harlan initially suspects coyotes, but his examination of the carcasses leads him to rule that possibility out. Larry, desperate to protect his herd, vows to shoot any animals that return, and he goes ahead with his plan, despite a warning from Harlan—only to be killed by a pack of dogs when the predators strike again. Soon after, a dog or two kill an SU student and an elderly lady, intensifying the town’s fear.
Harlan and Michael sift through the evidence and converge on a disturbing hypothesis: the murders are being committed by dogs that have formed packs. Michael frames a bold theory that pheromones could cause animals of a given species to unite and act as one. The scent or signal, they suspect, may be emanating from a top-secret government experiment being conducted at a nearby linear accelerator. They share this troubling possibility with the SU president, Dr. Martin Koppelman, Sterling Swanson, who refuses to entertain their warning and declines to issue any precautionary measures. Undeterred, Harlan resolves to issue a warning himself, a move that could cost him his job.
Meanwhile, a local kindergarten dog show provides a vivid demonstration of the threat: the dogs in attendance begin to pack and attack people, leaving no one unharmed. The town’s fear expands as more people confront the reality of dog packs roaming at night. A posse forms, led by Sheriff Jimmy Goodman, Eric Server, who initially resists, then joins the effort as the danger becomes undeniable.
Harlan and Michael attempt a dramatic test inside a plastic-enclosed setup: if their pheromone experiment works, they plan to flood the area with a stronger signal to break up the packs. But just as they prepare to begin, a dog explodes through the enclosure, colliding with a nearby pack. Back outside, the posse is overwhelmed; Jimmy fights to retreat toward safety but is hunted down. He survives a brutal chase only to be fatally wounded when a dog corners him at his car, forcing him to shoot the creature before collapsing.
Lounging in the shadows of tragedy, Dr. Charles Aintry, Dean Santoro, invites fellow professor Charlotte Ingle, Linda Gray, home after a date. She declines his advance and heads inside, coldly signaling the end of their budding connection. While Charlotte later takes a shower, she is brutally attacked and killed by a dog, underscoring the indiscriminate terror now gripping the town.
Harlan and Michael hurry to the police station and discover a dead man as they ask for help. They radio for Jimmy, but receive no reply, so they arm themselves and head out. As they pass Koppelman’s house, they witness him being attacked by a dog. Desperately, Koppelman begs for help, and Michael fires at the attacking animal, but his shot mortally wounds Koppelman instead. Harlan shoots the dog, ending its threat, while Michael, shaken by the unintended kill, breaks down in shock. The two later discover Mrs. Koppelman dead in the swimming pool, a grim sign of how quickly danger spreads.
Michael vows to warn the students, and Harlan checks on Caroline, urging evacuation. As distant howls echo in the night, Caroline’s own dog trembles with fear. Back on campus, Michael gathers students in the dorm lobby and urges them toward safety in the library, promising evacuation at dawn. Most students doubt him, but a few reluctantly follow. A student named Cory hears Howard Kaplan, Barry Greenberg, hungry and stumbling toward the cafeteria, and Michael arrives just in time to direct him to throw food to the dogs, attracting some of the pack away from hard targets and buying a moment of safety. Howard obeys, the plan buys a brief pause, but the danger is relentless, and the encounter leaves Michael with a growing sense of responsibility and failure.
As night deepens, the dorm’s occupants attempt to reach the library, while Caroline and Harlan take shelter in Caroline’s house. The howling rises in the distance, and Caroline’s dog trembles with fear as the danger draws closer. Eventually the dogs break into the home’s garage and begin tearing through a doggy door as Harlan blocks the path with a makeshift barricade. The pack overruns the chamber and advances, but a final desperate stand gives way to chaos as the dogs flood the space. Caroline and Harlan flee to the car with her dog, and they narrowly escape, halting only as one more door yields to the pack’s assault.
Back at the campus, Michael and Howard reach the dorm’s vicinity only to find two of the students dead. Howard panics and runs toward the library, while Michael warns him to stay cautious. A large dog pounces on Howard, shattering the glass door and allowing the pack to flood inside. The students inside the library are overwhelmed, leaving Michael with the grim realization that his warning was poorly timed and insufficient to evacuate the entire campus.
Harlan and Caroline manage a high-tension escape by car, and they encounter a college police vehicle on the road, discovering three corpses, including Michael. The radio cheers a grim update: dog attacks of a similar nature are erupting in neighboring towns, and dogs are described as the only species involved in the incidents. As the car pulls away, a further, chilling note arrives: a cat’s ominous meow punctuates the threat, suggesting danger may still lurk beyond the horizon.
