The Moonlight is a slave ship involved in the highly lucrative trade of human cargo. As Jessie discovers after being kidnapped and taken aboard the ship, the Moonlight is heading off to Africa, where Captain Cawthorne will buy as many slaves as he can before returning via the island of São Tomé to Cuba, where the slaves will be sold to a Spaniard.

São Tomé is a Portuguese-held island off the coast of Africa near the equator. At one point in the story, when the ship is running low on provisions, Jessie is relieved to know that the Moonlight will soon be setting sail for the island, where it will take on much-needed food and water.

The water ration has recently been reduced again, and poor Jessie goes about his duties each day with a dry mouth. And what duties they are! Instead of searching for rats in the hold, Jessie has graduated to empty the slop buckets.

Jessie may be glad that the Moonlight will soon be heading towards São Tomé, but the ship's journey is very far from being over. There are still many more miles to travel, and it is reckoned that it will take another three weeks before the ship eventually reaches its destination in Cuba.