While enjoying a vacation filled with adventure, scuba-diving enthusiasts David Sanders (Nick Nolte) and his British girlfriend Gail Berke (Jacqueline Bisset) discover a plethora of artifacts near the sunken ships of Bermuda. Among their finds is a fascinating ampule containing an amber-hued liquid and a striking medallion adorned with the image of a woman, alongside the intriguing letters "O.P.N.S.C"—an abbreviation of "Ora pro nobis, Santa Clara" and marked with the date, 1714. This discovery sets off a thrilling chain of events as they seek insight from the seasoned lighthouse-keeper and treasure-hunter Romer Treece (Robert Shaw), who correctly identifies the medallion as of Spanish origin and becomes captivated by their quest.
Unbeknownst to the couple, the ampule captures the attention of a dubious figure—the man who rented them diving equipment. This leads to a tense encounter with Henri Cloche (Louis Gossett Jr.), a local drug lord, who attempts to buy the ampule. When the couple refuses to part with it, Cloche ominously resort to intimidation using elements of Haitian black magic to instill fear.
Further examination reveals that the ampule contains medicinal morphine from the Goliath, a ship that tragically sank during World War II while carrying munitions and medical supplies. The Goliath wreck is notoriously treacherous, deemed off-limits due to the risk of explosive remnants. However, Treece surmises that recent storms may have revealed not only the morphine cargo but also an ancient Spanish treasure nearby. Crafting a cunning plan, Treece negotiates a deal with Cloche, convincing him they will retrieve the ampules together, while secretly plotting to uncover the treasure.
As the days unfold, the trio—Sanders, Berke, and Treece—embark on numerous dives deep into the underwater ruins, successfully retrieving countless morphine ampules from the Goliath and additional artifacts from the mythical Spanish wreck. They are joined by Adam Coffin (Eli Wallach), the only survivor from the Goliath, although his motives appear murky, especially when he inexplicably claims he "probably fell asleep" amidst a shark attack.
Delving into Treece's extensive library, they piece together the saga of the lost treasure ship, identifying valuable artifacts, including a significant metallic jar etched with the letters "EF." They even uncover the identity of the noblewoman Isabella Farnese for whom this treasure was crafted. Sanders is unwavering in his desire to retrieve at least one valuable item to validate the treasure's worth, aware that its significance hinges on provenance. Meanwhile, Treece contemplates eliminating the Goliath to thwart Cloche's grasp on the morphine, leading to increasingly dangerous confrontations.
Tragedy strikes when Treece's associate Kevin (Robert Tessier) is brutally murdered by Cloche's goons. The chaos escalates further when Coffin, revealing his treachery, meets his demise activating a deadly booby-trap while attempting to claim the precious morphine for himself. In a riveting climax, an epic clash unfolds during the final dive, where Cloche and his crew are defeated by a fearsome moray eel dwelling in the wreckage. This intense conflict results in the Goliath's destruction when its volatile cargo ignites.
Despite the thrilling turmoil, Sanders, Berke, and Treece emerge victorious, recovering a magnificent gold dragon necklace that ultimately provides the crucial provenance needed to validate the legendary treasure they have uncovered, marking the end of their perilous but rewarding adventure.
