The film follows Robert Maynard, a British Navy lieutenant, as he volunteers to earn a reward by proving that the privateer Henry Morgan also dabbles in piracy. To get the evidence he needs, Maynard goes undercover as a surgeon aboard the ship of pirate Charles Bellamy, whom he suspects is in league with Morgan. On board, Maynard is joined by fellow spy Briggs, and together they uncover a shocking truth: the notorious pirate Blackbeard, Edward Teach, has murdered Bellamy and seized command.
Also aboard is Edwina Mansfield Edwina Mansfield, the daughter of a pirate who had been betrothed to Bellamy. Blackbeard knows that Morgan loves Edwina and will do whatever it takes to reach her, adding a personal edge to the treachery at sea. Blackbeard orders Maynard to extract a bullet from his neck, and assigns a loyal crewmate, Gilly, to watch over the undercover officer. Gilly, secretly wary, slips Maynard a note begging him to slit the pirate’s throat, but Maynard resists, choosing caution over impulse.
Maynard makes a bold move, slipping into Blackbeard’s quarters and discovering Bellamy’s logbook, which he hopes will reveal Bellamy’s supposed alliance with Morgan and any stolen treasure. A delicate duel of wits unfolds as Maynard defends Edwina against an unwanted advance by a lecherous pirate, killing him with his dagger. Edwina confides that she agreed to marry Bellamy to escape Morgan, and that she has hidden treasure somewhere in a clothes chest she once shared with Bellamy.
Blackbeard breaks open one of Edwina’s chests but finds only letters in which Edwina implicates Morgan as Bellamy’s ally. Maynard tries to steal the incriminating letter, but Blackbeard stops him, warning that Morgan’s arrest would release all of the loot to the King. Blackbeard ultimately identifies the treasure chest and claims its contents for himself. In response, Maynard sabotages the ship’s water supply, forcing Blackbeard to beach the vessel for repairs. The pirate sees a chance to bury the treasure, but the plan falters when he murders the sole witness and hides the treasure beneath a rock rather than burying it properly.
On land, a derelict who bears a striking resemblance to Blackbeard is found, and Morgan’s men decide to protect him, sensing he could be useful later in the game. Meanwhile, Henry Morgan and his men stealthily come ashore at night and slay twenty members of Blackbeard’s crew. Blackbeard then disguises himself by swapping clothes with the derelict, sending the real lookout to face Morgan’s wrath.
Edwina and Maynard reach port only to discover the very man who gave Maynard his assignment is behind bars, and Morgan has become Governor. Morgan places a price on Maynard’s head to keep him from exposing the double game of Morgan’s faction. The lovers’ only option appears to be stealing Blackbeard’s captured ship and fleeing together, but the real Blackbeard arrives, raiding the local jail for fresh recruits and complicating their escape.
A fierce sea battle ensues when Morgan wheels a captured Spanish galleon into combat, hoping Blackbeard will attack. Blackbeard drags Edwina into the mêlée as a human shield, and Morgan is forced to disengage the fight to save his own men. The battle leaves heavy casualties, and first mate Worley urges the crew to return to the treasure. When they finally find it, Worley is killed by Blackbeard, and the remaining crew members are locked below.
Gilly, however, knows a network of secret passages that Blackbeard doesn’t, and that knowledge becomes crucial as the mutiny of the sea drums on. In a dramatic finale, Blackbeard is captured and buried up to his neck on the beach, his fate sealed by the rising tide. Maynard and Edwina seize a dinghy and escape while the wreckage of vengeance swirls around them, deciding that any place away from Morgan’s treacherous world is better than the life they left behind.
