Errol Flynn plays Peter Blood, a former military man and doctor who is accused of treason because he treated the injuries of a rebel. As a result, he's sent as a slave to Jamaica. There he forms a motley crew of other slaves (all white) and steals a ship. Within a couple years they're the most infamous pirate crew in the Caribbean.
There are movies that are "classics" that still hold power today ("Casablanca" comes to mind, also directed by Michael Curtiz). This is not one of those movies. It was very successful in the theatres and it's the movie that made Flynn a star, but from a remove of 80 years all I really see is a handsome man posturing and posing. The most memorable pose is, of course, the poster-ready noble-looking-skywards-profile, and he does it so well.
The movie is by modern action movie standards incredibly slow-paced: it's more than an hour before there's anything that could conceivably be called a physical altercation. Prior to that we're establishing characters and politics ... and not doing it terribly well as the acting isn't good. I was also fascinated by the whole "righteous man" / "wrongly accused" thing, and how that fits with being a pirate.