With Carter off on a military mission, there's a new sheriff in town, and Fortune, Ida Belle, and Gertie have been tasked with not interfering. But then a young girl is pulled out of the swamp, unconscious and with no memory of how she got there. And when no one comes to claim her, Fortune knows they have to help.

For decades, a mysterious group known only as the Brethren has been rumored to live deep in the swamp, their contact with civilization rare. Some suggest the girl might have been one of them, but if so, why was she alone in the swamp in the middle of the night? Swamp Team 3 is on the case, but long-buried secrets like to stay that way, and finding the Brethren proves to be a difficult and dangerous task. The deeper they dig, the more pieces of the puzzle they expose, but can they put the pieces together in time to save the girl?