Chronicling the McKenna brothers' childhood in western Colorado during the 1950s and 1960s, Dennis writes of his adolescent adventures including his first encounters with alcohol and drugs (many of which were facilitated by Terence), and the people and ideas that shaped them both. Brotherhood of the Screaming Abyss weaves personal narrative through philosophical ideas and tales of psychedelic experimentation.

In this book, Dennis describes these inquiries with the wisdom of perspective. In his account of what has become known as "The Experiment at La Chorrera"— which Terence documented in his own 1989 book, True Hallucinations — Dennis describes how he had visions of merging mushroom and human DNA, the brothers' predictions for the future, and their evolving ideas about society and consciousness. He also offers an intellectual understanding of the hallucinogenic effects of high-dose psychedelic mushrooms and other psychedelic substances.

Dennis, now world-renowned for this ethnobotanical work, describes in Brotherhood his early interests in cosmology and astrology, his sometimes rocky relationship with his older brother and how their paths diverged later in their lives. Dennis describes his academic career in between touching accounts of both his mother's and Terence's battles with cancer. In the 10th Anniversary edition of Brotherhood, Dennis reflects on scientific revelations, climate change, and the social and political crises of our time.

The new edition also features both the original foreword by Luis Eduardo Luna and a new foreword by Dr. Bruce Damer. Brotherhood of the Screaming Abyss is a story about brotherhood, psychedelic experimentation, and the intertwining nature of science and myth.