In August 1943, Ben Staves arrives on a tiny island off Cuba to join an in-progress Navy test aimed at developing a shark repellant. The secluded research outpost has been led for ten months by Lt. Cmdr. Leonard Evans, a dedicated ichthyologist formerly with the Scripps Institution, who is backed up by Ensign Dunk Duncan and a diligent cameraman, Chief Petty Officer Gordy Gordon. Staves explains he’s there to help, not to seize control, but the veteran scientist is wary of hurried leadership from a man still trying to reconcile battle trauma with science.
When Evans proposes a slow, methodical pace, Ben presses with a blunt challenge: < > "Why not now?" > The exchange sets the tone for the dynamic between two men with different temperaments but a shared goal. The team heads out into the bay on a small fishing boat, aided by a local skipper and a teenage deckhand, Carlos. The initial tests focus on copper acetate, a method that shows promise at first but quickly fades as the protective cloud dissipates, underscoring the stubborn difficulty of keeping sharks at bay in real-world conditions.
As the days stretch into weeks, Ben reviews Evans’s meticulous reports and grows restless with the perceived slowness of progress. Evans defends his careful approach, arguing that their work remains only 80% effective because they lack human test subjects, while Ben pushes to pursue parallel avenues. The conflict sharpens but remains framed by a common stubborn resolve: to prove the concept before the Navy moves forward. In a bid to accelerate results, Ben insists on a seven-day-a-week schedule, even as his own life back home—his gorgeous young wife Martha—remains distant in Havana.
Two weeks into the renewed push, the team posts 72 consecutive positive results. Elated, Ben urges submission to naval channels, but Evans cautions that they have not hit the 100-test target. Ben invokes his authority to push ahead, and Evans, though reluctant, agrees to proceed. Disaster looms when Carlos, in a moment of playfulness, trips and falls overboard. Sharks descend as the crew calls for him to swim toward the still-standing repellant cloud. He does, but panics and pierces a nearby shark with his knife, drawing a feeding frenzy that finishes him off before the rescue boat can haul him aboard.
After the brutal setback, the group steels themselves to test another candidate: octopod ink. Duncan concocts a synthetic formula and blends it with their prevailing copper acetate mixture to slow its dissipation in water. Ben promises that he will volunteer as a test subject once the 100-fish target is achieved, signaling a rare moment of personal risk in service of science.
Ben travels to Havana to seek two expert riflemen to join the operation, where the naval attaché raises suspicions and pressures him to obtain Navy approval before proceeding with the plan to use human subjects. The attaché warns that such a move requires proper authorization, reframing the experiment within official channels. Undeterred, Ben and Evans push ahead with refinements to the reagent and delivery method.
The ink-based tests show strikingly positive results, encouraging the team to plan one final test the following day, albeit still short of the agreed-upon 100 trials. In the water, Ben begins to dissolve a compact cake of repellant by waving it around, a dramatic demonstration of the method’s potential. Yet the sequence eerily echoes earlier near-misses: a similar test without the squid ink had failed when Carlos’s panic sparked a feeding frenzy. Now, with fresh confidence, the team watches as a new group of sharks approaches Ben.
A lookout reports a larger pack converging, and the atmosphere tightens as tension mounts. A marksman fires at a shark, drawing blood into the water and igniting the feeding frenzy. As Ben makes his way toward the boat, Evans unloads a torrent of extra repellent from the barrel—hundreds of times more than a downed flier could release—creating a dense, protective mist that units the crew against the predators. Ben is hoisted aboard just in time, drenched in relief and a shared sense of relief, beer in hand, contemplating the likelihood of enduring danger with renewed resolve: he intends to drink several more in the days to come.
The event underscores the fragile balance between bold experimentation and cautious science, and it leaves the men to weigh the cost of progress against the ever-present threat of the sea. The story is framed by the steady narration of Charles Collingwood, whose voice threads through the film to anchor the scientific drama in a broader human context, while the principal figures—[Ben Staves], Evans, Dunk, Gordy, Martha, and Carlos—navigate a complex bond of professional duty, personal longing, and the relentless pursuit of a life-saving breakthrough.
