Mary is orphaned by the Black Plague by age 8, her entire family having been carted off after death to be sold as doctors' dissection models. For a few years she makes her way as part of a beggar's gang of children on the streets. When her best friend Charlie is found dead in a back alley, she takes his clothes and his knife and decides to try something different.
A ship named the H.M.S. Dolphin is leaving port and Mary manages to gain a place on board due to her reading skills and her charade as a boy, "Jack." Along with the five other ship's boys, Jacky spends the next two years on board the ship as they travel around the North African coast and the Carribean coasts. There are plenty of pirates, bullying sailors, violent battles, and always the fight to keep her true identity undercover.
For those who remember reading The True Confessions of Charlotte Doyle, this is similar, while a little formulaic. But it was a treat, and there are sequels!-- I plan to read every one.