The action starts when Marlowe, a former pirate now living as a wealthy planter in the Virginia tidewater near the new capital, Williamsburg, is fitting out a privateer called the Elizabeth Galley to prey on Spanish shipping. War with Spain has broken out -- it's the early 1700's. A subplot from the previous book had Marlowe freeing the slaves he inherited along with the plantation, and he'd trained some of them in seamanship. He sends a crew of these freedmen to Newport News to deliver some equipment to another plantation he owns. They discover a slaver near the entrance to Chesapeake Bay and she's in distress. They come alongside and James, the freedman commanding the river sloop, loses his temper, remembering his time as a slave being transported to the New World, and kills the captain.
He knows he can't go back to Williamsburg but he sends the sloop back to inform Marlowe, while James takes over the slaver and tries to sail it back to Africa. Marlowe meets the Royal Governor and tells him the story. No letter of marque and reprisal, but an order to pursue the slaver, recapture Freedman James, and bring him back to be hanged.
At this point there are both authentic strands in the story and hard to believe elements. While the story holds your interest and has a lot of action, both seafaring and on shore, it's pretty hard to believe that James the ex-slave, even with help from one of the slaver's officers, could shape a good course for Africa after learning his seamanship on Chesapeake Bay. While Elizabeth Galley is a fast sailer and the slaver is badly damaged, having the former find the latter in the enormity of the Atlantic just beggars belief. The battleship Bismack evaded half the Royal Navy, equipped with airplanes and radar, for most of a week in World War II. Of course everything moved slower in 1710 but that meant you had to know where the quarry was going if you didn't have enough ships to form a scouting line. No way Marlowe could guess where in Africa James picked as the destination -- and the slaves' leader, member of the Krooman tribe that I know from earlier literature (spelled differently here), double crosses James and steers for Whydah, the slave port, which James wants to avoid.
However shaky the historical background, the story is fast moving and has no obvious technical deficiencies (keeping in mind I'm not as familiar with 18th century seafaring as I am with 19th).
It's an entertaining read, worth your time, and some of the tribal cultures are portrayed believably, probably even accurately.