Isabel forged into the steamy jungle. Instantly, her world transformed from one of brisk, salty breezes and sweeping horizons to muggy, stagnant air and dense, stifling green. An orchestra of birds scolded her as they fluttered from branch to branch, squawking their protests at the intruders who dared to invade their domain.

Up ahead, the plume on Morris' tricorn no longer ruffled in the breeze but wilted as it flopped up and down with each thud of his boots. In front of him, ...