"The sea is slavery," begins Fred D'Aguiar's powerful novel, which starts aboard the Zong, a slave ship returning from Africa in 1781. Only ten weeks into the voyage, the Zong is struck with a disease that threatens to infect all of the human cargo.

The ship's profit-driven commander, Captain Cunningham, conceives a gruesome plan to safeguard his financial investment. In order to recover insurance money and protect the rest of the valuable stock, the captain orders his men to bring the sick and infirm slaves on deck in small numbers and throw them over the side.

During the roundup, Mintah, a young African woman raised in a Christian mission, begins to taunt Kelsal, the first mate, in whom she sees a hint of humanity. But her scheme fails, and Mintah is hurled into the sea, along with 131 other slaves.

Then, almost by miracle, Mintah is able to grab the ship's rigging and climb back on board, where she hides out in a supply room. From there she tries to rouse the remaining slaves to rebel, becoming a secret force on the ship and stirring up unease among the crew with a voice and a conscience they seem unable to silence.