Off the coast of a picturesque Caribbean island, a young couple, Lou and Lisa, make a hasty escape from their hotel under the cover of night, wanting nothing more than to indulge in a romantic swim. Little do they know, their aquatic escapade leads them to a haunting sunken wreck that serves as a deadly lair for piranhas. In a tragic turn of events, both lovers meet a gruesome fate, devoured by the merciless fish lurking within the wreck.
The following day, a group of intrepid tourists, including Tyler Sherman, embark on diving courses organized by Anne Kimbrough, who is an employee at the Hotel Elysium. Things take a sharp turn when one of her divers disregards her strict warnings and swims into the forsaken wreck. As she rushes back to the surface, she soon discovers the horrifying truth when the mutilated remains of her 'missing' student are recovered from the depths.
Meanwhile, Anne's estranged husband, Steve, a seasoned police officer, dismisses her pleas to examine the body, believing that her worries are unfounded. However, Anne's instincts are sharp, and she recognizes that the patterns of death do not align with any known marine threats in the area, raising alarms in her mind regarding the true nature of the danger at hand. With a history of friendship between Steve and local dynamite fisherman Gabby (Ancil Gloudon), the two join forces to uncover the mystery of these suspicious deaths.
As tensions mount, the guests at the hotel find themselves tangled in romantic pursuits; among them, a middle-aged widow named Beverly flirts with the younger dentist Leo. A sudden arrival of two adventurous young women, Jai and Loretta, sets the stage for a series of events that lead to chaos. Declaring themselves as "sea bandits," Jai becomes intrigued with the kitchen and playfully interacts with Mal, a cook with a mental disability. But their mischief takes a dark turn when, after stealing from Mal, they sail too far out into the water, unwittingly inviting the piranhas' wrath. The night takes a horrifying turn as Jai suffers a fatal attack, leaving her and Loretta fighting for their lives against the relentless piranha swarm.
In a different light, Anne is increasingly troubled by her surroundings and decides to involve Tyler further in her inquiries. They visit the morgue, revealing her background as a marine biologist before her marriage to Steve. There, she captures evidence of the gruesome nature of the death her diver experienced. Amidst her investigations, a terrifying revelation occurs when a nurse falls victim to a flying piranha that escapes into the night after gruesomely attacking her.
In the aftermath, Anne's personal life and professional urgency collide, leading to a brief romantic encounter with Tyler, further complicating her already tumultuous relationship with Steve. Tempers flare when Steve discovers the truth about Anne's findings as well as her intimate liaison with Tyler, resulting in a confrontation filled with accusations and misunderstandings regarding Anne’s opinions on the emerging threat.
Despite her attempts to raise awareness, Raoul, the pompous hotel manager, disregards Anne's warning and ultimately dismisses her. As the lurking piranhas grow more emboldened, Gabby's son becomes another victim of the situation, igniting a fierce desire for revenge against the creatures that have invaded their lives. As the resort hosts a fish-catching party, the piranhas strike back, launching an assault on terrified guests, forcing Anne to lead the survivors to safety.
The stakes soar as light breaks and the piranha retreat, revealing their vulnerability to daylight. Anne and Tyler hatch a daring plan to utilize explosives in an attempt to vanquish the plaguing fish for good. Yet, their children, Chris and Allison, unknowingly venture into hazardous territory while seeking thrill on the water, putting themselves in grave danger. As Chris and Allison's boat becomes stuck, their fates hang in the balance.
Racing against time, Anne and Tyler don scuba gear and journey into the sunken wreck to plant explosives, but they soon face peril as they are cornered by the waiting piranha. Back on the surface, Steve makes a desperate attempt to help, and as the clock counts down, Tyler bravely sacrifices himself so that Anne can escape. In a nail-biting conclusion, Anne narrowly survives the explosion that destroys the ship and eliminates the piranha threat once and for all. As she surfaces, she finds safety and solace among Steve and the two children, reclaiming a moment of peace after the harrowing battle with nature's most vicious predators.
