In 1806 the English privateer Port au Prince anchored in the South Pacific kingdom of Tonga, seeking food and help. Instead, they were attacked by the Tongans. Half the crew was massacred and thrown to the sharks; the rest were enslaved. Fourteen-year-old William Mariner was adopted into the family of Finau 'ulukalala, the powerful and charismatic king of the islands. Mariner spent four years in Tonga, participating in Finau's wars, managing a plantation, and learning all he could of the language, customs, history, and politics of the Tongan people. After he returned to England in 1810, he...