Little coastal town is being terrorized by deadly Barracudas.
- In the coastal village of Palm Cove, Florida, two government agents posing as land developers arrive in town.
On a stretch of nearby beach, Mike Canfield (Wayne Crawford), a marine biologist, tests for polluted waste from the town's manufacturing plant, Jack Chemical Company. As Mike dives to collect underwater samples near the drainage pipes offshore, the plant's owner, Papa Jack (Bert Freed), orders his son, Bubba (Jerry Rhodes), to detain the marine biologist and notify the Sheriff. When Mike is arrested for trespassing, he instructs an assistant to grab the samples tucked inside his Scuba fin so he will have proof of the plants environmental misconduct.
After spending the night in jail, Mike is treated with hospitality by Sheriff Ben Williams (William Kerwin) who plans to release him as soon as the judge is available. That day, a barracuda attacks and kills a young man while he is diving for lobsters, and later a young girl and her dog stumble upon the diver's severed head on the beach.
While investigating the scene, Ben tries to keep Floyd (Bob J. Shields), the local newspaper reporter, from turning the accident into front-page news. During a meeting with Papa Jack, Government Agent #2 (Rich Rhodes) warns that his organization, which represents a lucrative contract for the plant, is concerned about the breach by the marine biologist (Mike). Until a security evaluation is conducted, Papa Jack's delivery orders are on hold. Suddenly, Bubba rushes in to inform his father that thousands of dead, mangled fish have washed up on the beach. Papa Jack issues a hasty statement to Floyd that the waters have been thoroughly tested and this phenomenon is only temporary.
During lunch at the local diner, Ben and his deputy, Lester (Cliff Emmich), notice that several of the town's residents are behaving aggressively and assume the behavior is heat-related.
The next day, Ben learns that a second swimmer has died from a fish attack, and Mike returns to Palm Cove, with his girlfriend Liza Williams (Robert Leighton) to help with the investigation. Although water samples indicated trace elements of other chemicals besides the plants usual fertilizers and pesticides, Mike is reluctant to link the pollution to the accidents until he collects more evidence. Ben thinks the local doctor, Elliot Snow (Jason Evers), will be amenable to letting Mike use his lab for analysis.
The next morning before dawn, Ben pilots the boat offshore from the plant, and Mike dives for additional samples. After encountering a school of barracudas, he surfaces in a panic and instructs Ben to net some of the lethal fish for testing.
At the doctors lab, Mike studies the barracuda specimens, but Snow suggests that he is leaping to conclusions by trying to connect the chemical plant to fish activity. Meanwhile, Papa Jack receives a threatening phone call that there will be serious consequences unless he can contain the situation, which is now a front-page headline: 'Chemical Plant Creates Killers in the Deep'. After Mike identifies other trace chemicals like glucose, he takes a break and leaves the lab.
Mike is resting on the cot inside the jail, when Floyd arrives and pesters him about the latest developments in the case. Mike states that the plant is definitely responsible for contaminating the water, but the fish attacks are still under investigation.
Floyd quickly returns to the newspaper office, but along the way he is followed. After Floyd tells his assistant Toby (Scott Avery) that this story will make them famous, Government Agent #1 (director Harry Kerwin) enters and guns down the two reporters. Maggie Snow (Bobbie Ellyne Kosstrin), the doctor's daughter, also becomes a victim when she arrives to meet her boyfriend, Toby.
Meanwhile, Mike returns to the lab and suspects that Snow has tampered with the samples. While poking around the doctor's study, he discovers a file titled 'The Lucifer Project', which documents experiments that Snow has been conducting on patients, correlating low blood sugar levels and aggressive behavior using chemicals like glucose.
Realizing that the doctor is somehow connected to the pollution, Mike returns to the Sheriff's residence. Agitated, Mike describes to Ben how Snow is trying to control the behavior of the town's population. He explains that the doctor is injecting chemicals into human beings that prevent absorption of nutrients, leading to low blood sugar, a condition that encourages aggression. He adds that these chemicals, manufactured at the plant, must have been leaked into the ocean, causing fish to also be nutrient-deprived and go hungry. Because the barracuda is a heartier fish, it can survive by eating weaker fish, but eventually the barracuda begins to prey on humans to ward off starvation. If the evidence of the strange fish behavior had not surfaced, the plot might have gone undetected.
Grabbing his gun, Ben drives to the chemical plant to investigate, while Mike and Liza track down Lester and instructs him to radio for back up from the state police. Lester and Liza leave town to get help. In the process, Mike learns about the killing of the two newspaper reporters and Maggie Snow.
At the doctor's office, Mike confronts Elliot Snow, blaming him for Maggie's death and for using the town's population as guinea pigs. Snow confesses that the project was used as a war preparation tactic, to ensure that the populace would welcome aggression and not protest it. Clutching a photograph of his daughter, Snow realizes that the experiments must be stopped and reveals that the government is controlling the project.
At the plant, Ben discovers the murdered body of Papa Jack, and encounters the two agents who begin shooting at him. Mike arrives to assist Ben and a shootout happens. Soon, several state police cars arrive at the plant, and Ben orders the officers to take the two agents into custody. After the two agents throw down their weapons and appear to surrender, Agent #1 turns to the police cars, lowers his hands and shouts, "Fire!" The officers (in league with the government agents), point their weapons at Mike and Ben and shoot them both dead in a hail of bullets and buckshot.
(In some versions of the film, a on-screen text of the aftermath shows news reports which announce that the town of Palm Grove has been evacuated on account of an environmental hazard in the local waters. There is no mention of the barracuda attacks or of the murders in the town as well as the Lucifer Project.)