A young concrete worker meets two men in a tavern who share his desire to find the Darlington River's rogue branch. It appears only once a year for a few weeks when the moon is right. He meets a young, rich widow who shares the same interest in the random phenomenon and outfits the largest boat for their journey. The rogue leads to a laboratory and hybrid nursery beyond the tangle of marsh grass and vines. They break through the wall of green and arrive on a silver lake that has a treacherous whirlpool. What they discover there, is spine-chilling.