In control of the merchant ship, Yann closes in on his former pirate ship the Meduse, which has reached its destination, a Carib island shared by a lonely peddler in a tiny fort and the cannibal Indian village. There cabin boy Nicolas's heroic efforts to defend Agnes are no match for her fiendish husband count Hervé, so she must accept committing to an alliance with the marquis, who clearly betrays his partners anyhow. Mistrusting him too, she begs Nicolas to run ands seek help. He does so while bravely serving as guinea pig for the English-invented diving casket, having found indications the admiral's ship carrying the Kermelec treasure sunk there. Cha-Ka is determined to sacrifice his life for his lovers' in a shamanistic ritual, but she reproaches Yann for helping and thus forcing her to live without him, however the Indian offers a ritual alternative in a dream.