An Arab dhow is found half-sinking and abandoned - with 16 dead bodies sealed in the hold. Duval suspects it had been engaged in the slave trade and flies to Mombasa to examine it, only to discover the reason for its abandoning - its human cargo had been dying of plague. The crew of the boat make it back ashore, but those behind the trade are not taking any chances. When the body of a dead seaman - shot, but dying of plague - is discovered, Duval realises how ruthless they are.