The lush, overgrown banks of Massachusetts' Assabet River are the ideal place for Dr. Adam Walker to find coveted medicinal plants for his remedies. But on one balmy August morning he finds something very different. A stranger, identifying himself as Henry David Thoreau of nearby Walden Pond, approaches and entreats Adam to accompany him upriver. He has discovered the body of a young black man at the base of the cliff known as Devil's Perch. As they examine the broken corpse and the surrounding scene, both men become convinced that the unfortunate victim was dead long before he fell. Yet the coroner's jury insists otherwise, dismissing the matter as an accident. Angered by the injustice, Adam and his lovely cousin Julia Bell agree to assist Thoreau in investigating. Adam notes in his new friend all the makings of a great detective an encyclopaedic knowledge of the natural world, uncanny observational skills, a sharp instinct for detecting human foibles. As the case progresses, the mysteries only deepen and there is no mistaking the brutal slaying of a womanizing army captain as anything other than the coldest murder. Journeying from their tranquil village to Boston's most disreputable district, they gradually uncover the monstrous truth even while a vicious killer prepares to end their inquiry for good.