Rafael Sabatini's novel comes to life as an Irish doctor is sold into slavery in Jamaica but transforms enslaved rebels into pirates, who defeat the French after King James is replaced.
In 1685 England Jeremy Pitt (Ross Alexander) comes for Dr. Peter Blood (Errol Flynn), who has retired from war. Blood treats a wounded rebel, is arrested and tried for treason against James II, who commutes the rebels' hanging sentences to enslavement in the Caribbean. Arabella Bishop (Olivia de Havilland) asks her father Colonel Bishop (Lionel Atwill) to buy them all to keep Dixon from getting them. She buys the proud Blood. A slave who tried to escape is branded on the face. Arabella has Dr. Blood treat the governor's gout, and he gets the two out-of-work doctors to help him buy a boat. Blood organizes the escape and has debtor Nuttall buy a boat. Arabella gives him an alibi when her father questions him. Peter tells her why he resents her, apologizes, and kisses her; but she slaps him. Jeremy is whipped to talk. Dr. Blood treats the governor and then their navigator Jeremy. Bishop has Blood tied up; but a timely attack by Spanish ship cannons and pillaging enables Blood to lead slaves to the boat, which has been sunk. Blood and his men take over a Spanish ship. Spaniards demand money from Port Royal. Blood's men use cannons against the Spaniards in the boats. Bishop goes out to the ship, finds his money, and says he is grateful; but they throw him overboard and set sail as pirates.
Captain Blood dictates an agreement of shared brotherhood with punishment for hiding treasure, being drunk on duty, and abusing captured women. Blood terrorizes the Caribbean, and King James makes Bishop governor. Blood pays equal shares plus compensation for lost limbs or bravery. At Tortuga he agrees to a partnership with French buccaneer captain Levasseur (Basil Rathbone) under his articles. Arabella sails from England with Lord Willoughby (Henry Stephenson), who is hunting Blood. At Virgen Magra Levasseur holds Arabella for ransom; but Blood buys her for pearls. Levasseur and Blood fight with swords, and Blood kills Levasseur. Arabella refuses jewels from Blood, whom she calls a thief. He sails for Port Royal to return her; his men refuse but realize they have no better plan without his leadership. Seeing a sea battle at Port Royal, Willoughby informs them England is at war with France and asks them to fight for the king. The men laugh and reject his offer to join the navy until they learn that William has replaced James as king of England.
Arabella is put in a boat for safety. Blood uses a French flag to get close, destroys a French ship, and boards a second, as their ship is destroyed. Blood's men take over the French ship. Arabella tells Peter to leave, because her father is returning; but Willoughby has removed Bishop for abandoning his post and has made Blood governor. Arabella pleads for her uncle and kisses Peter.
History gives this swashbuckling adventure context. Circumstances give the ex-slave rebels an excuse for piracy, and a changed government a reason to return to duty, portraying a rough world, where one form of violence or another tends to dominate.