The Thundering is the story of psychologist Cathy Morgen, who encounters the most challenging case of her career very early one morning in 1961 at Summerhaven Mental Hospital. Joseph, a teenage boy who has been admitted on an emergency basis, is behaving exactly like a frightened horse -- kicking, neighing and biting. What has caused this apparent delusion? Can Joseph be restored to humanity? Cathy has no answers, but she is determined to find them. Not only does she feel compassion for Joseph, but she hopes that treating him will help her overcome her guilt at failing to save a similar young patient. Assisting Cathy is her flighty teenage daughter Suzie, who is entranced by the idea of helping Joseph. Opposing Cathy and her attempts to solve the mystery of the horse-boy are an arrogant psychiatrist; Summerhaven's genial but deceptive director, and Cathy's own husband, Dan, who longs for a conventional marriage.

Attempted escapes, brilliant insights, shocking confessions and unthinkable acts of betrayal all play a role in helping Cathy discover Joseph's traumatic past and heal his broken psyche. She will pay a significant price for his return to the world of humans.