Twenty-six-year-old Sir Oliver Tressilian is a wealthy young man who lives on the beautiful estate of Penarrow in Cornwall with his half-brother, Lionel. Oliver has successfully wooed Rosamund Godolphin, who lives in Godolphin Park, the estate adjacent to Penarrow. Seventeen-year-old Rosamund has agreed to marry Oliver, but her older brother, Peter, and Rosamund’s guardian, Sir John Killigrew, dislike the Tressilians and are opposed to the marriage. Oliver is in no hurry to get married and is content to wait until Rosamund is older and doesn’t need her guardian’s approval.
Peter visits Oliver and repeats the insults of Sir John, calling Oliver a pirate, slave dealer and murderer, which manipulates Oliver into a duel with Sir John. Oliver confronts Sir John, who is overly eager to fight. Peter’s scheme backfires, and Sir John is seriously wounded in the duel, leaving Peter even more resentful of Oliver. Oliver goes to Rosamund’s estate to tell her of the duel before she hears of it from her brother. Oliver promises Rosamund that despite the animosity between he and Peter, he will avoid her brother and never hurt him.
Oliver coddles his 21-year-old brother, Lionel, and provides generously for him. When Lionel begins a relationship with a woman with a bad reputation, Oliver discourages Lionel, and the brothers argue over the matter. While out for a ride one afternoon, Oliver stops and talks amiably with some local men, until he sees Peter.
Peter is drunk, and he and Oliver insult each other. In a drunken rage, Peter strikes Oliver with a lash, cutting his face. Peter then challenges Oliver and rides away. At first, Oliver pursues Peter, but after remembering the promise he made to Rosamund to never hurt her brother, Oliver goes home.
Lionel returns home later that night — bloody, frightened and exhausted. The woman he had been seeing has also been seeing Peter, and when Lionel found out, he went to confront his rival, only to meet him on the road. Lionel killed Peter in a duel, but since there were no witnesses, the death would be considered murder. Oliver loves his brother and decides to protect him by withholding the truth, even from Rosamund.
The next day when the news arrives of Peter’s death, immediate suspicion falls on Oliver. Oliver rushes to the Godolphin estate, but Rosamund refuses to see him. He pushes his way into her sitting room and sees that she too has believed the rumor. There is a trail of blood from the site of the duel that leads to Oliver’s home. Oliver refuses to tell Rosamund that Lionel is the killer.
Lionel knows his brother is widely believed to be Peter’s killer, but he does nothing to disprove the accusations. Instead, he becomes increasingly fearful that Oliver will eventually come forward with the truth. Even before Peter’s death, Lionel had daydreamed about owning Penarrow and all his brother’s assets, and so he begins to plot.
Lionel pays a sea captain, Jasper Leigh, to kidnap Oliver and sell him into slavery. After Oliver is kidnapped, everyone believes that he fled on his own to avoid criminal charges that were to be filed against him. Aboard the ship, Jasper Leigh tells Oliver of Lionel’s treachery and requests more money to return Oliver to Cornwall. Oliver agrees, but before they can go back, Spaniards capture the ship, and the crew is taken as slaves.
For six months, Oliver toils on the ship as an oarsman, naked and chained to his bench. Seeing how the Spaniards, who consider themselves Christians, treat their slaves, Oliver renounces Christianity. Oliver befriends his oar mate, a young Moorish man, Yusuf-ben-Moktar, who works diligently to convert Oliver to Islam.
Asad-ed-Din, the Basha of Algiers, a famous Muslim corsair and Yusuf’s uncle, attacks the Spanish galley. Oliver and Yusuf help defeat the Spaniards and free the other slaves aboard the galley. Oliver’s fighting prowess and Yusuf’s testimony grant Oliver special privileges in Muslim society.
When given the choice between converting to Islam and going back to slavery, Oliver chooses conversion. Oliver becomes the corsair known as Sakr-el-Bahr: the hawk of the sea. Soon Oliver is Asad’s commander in chief, conquering many ships and bringing back great wealth to Algiers. Since captured slaves are the property of the state, Oliver rescues English slaves by buying them himself and finding ways to send them home again.
Two years after being kidnapped, Oliver captures a sailor, Pitt, from Cornwall and talks to him about what has happened in his absence. Oliver writes Rosamund a long letter professing his innocence and sends it with Pitt back to Cornwall. Three months later, Pitt writes back that Rosamund burned the letter without reading it. The news sends Oliver into despair. Afterward, he hardens his heart and stops freeing English slaves.
Three years later, Oliver receives another letter from Pitt with news that Oliver’s conversion to Islam has rendered him legally dead by law, turning over his entire estate to Lionel. Furthermore, Lionel and Rosamund are engaged to be married, uniting the two neighboring estates. Two weeks later, he captures a Spanish vessel with Jasper Leigh at the oars. Oliver forms a plan.
With Leigh’s help, Oliver sails back to Cornwall, incapacitates Sir John and kidnaps Lionel and Rosamund. Oliver takes them back to Algiers. While he wants to keep them, both are property of the state and must be sold as slaves. The Basha, Asad, sees Rosamund, becomes deeply infatuated with her and intends to buy her for himself. He leaves a servant in the slave market to make the purchase, but Oliver outbids him. Oliver also buys Lionel.
When the Basha tries, unsuccessfully, to buy Rosamund from Oliver and then tries to take her by force, Oliver quickly marries her, foiling the Basha’s plans and infuriating the man at the same time. With the looming threat of the Basha forcibly taking Rosamund, Oliver realizes the danger he has put her in. He regrets kidnapping her.
Oliver tricks Lionel into admitting the truth about Peter’s death to Rosamund. Rosamund is horrified, and Oliver condemns Lionel to serve as a rower on his galley. Oliver sees the past from Rosamund’s perspective, and his anger toward her dissolves.
He still loves Rosamund and vows to return her safely to Europe, even if it means giving up his life for hers. When leaving for a mission to capture a Spanish treasure ship, Oliver smuggles Rosamund aboard his galley. To Oliver’s dismay, Basha accompanies him. The man is still consumed with lust for Rosamund.
Basha’s scheming son, Marzak, discovers Rosamund's hiding place aboard the galley. When the Basha attempts to take her by force again, Oliver threatens mutiny, but the crew would be equally divided between both leaders. An uneasy stalemate develops as the galley prepares to ambush the Spanish treasure ship. Instead, the first ship they see is an English ship belonging to Sir John, who has vowed to rescue Rosamund and Lionel, and to kill Oliver.
Seeing this as an opportunity to return Rosamund safely to England, Oliver throws Lionel overboard with a message for Sir John. The English ship tries to attack, but the Moorish galley, with its oars, is faster. To prevent the galley’s escape, Oliver hijacks command by threatening to detonate a powder keg.
The English ship comes alongside and boards the galley. Jasper Leigh stabs Lionel, who is first to board the ship. Oliver stops the fight and negotiates a truce. He will surrender himself if the Moors will return Rosamund to the English. Then the English will leave with Oliver and Rosamund.
The Basha is furious with Oliver for his treachery. He gives Oliver over to the English, believing that he will be hung. A truce is reached, and the ships depart. Oliver is flung into a hold to await execution. Rosamund, seeing that Oliver has sacrificed his life for hers, realizes she still loves him.
Lionel is aboard the English ship and mortally wounded. Sir John and other officers of the ship form a tribunal and pass judgement on Oliver, sentencing him to death. To their dismay, Rosamund defends Oliver and tells the story of what happened.
The men believe her mad or bewitched, until they get word that Lionel is conscious and asking for his brother. Lionel makes a full confession in front of the tribunal and asks Oliver for forgiveness, which Oliver gives.
Rosamund’s story is confirmed and the tribunal is forced to admit that they have no grounds to hang Oliver. He is told that he will be cleared of all charges and his property returned to him. Rosamund and Oliver embrace on the deck of the ship as they look forward to a future together.