鲍威尔（安东尼•霍普金斯 Anthony Hopkins 饰）是一位专门研究黑猩猩的人类学家，他在卢旺达长期观察观察当地黑猩猩的活动。然而两年前，这位性情温和的学者突然性情大变，杀了两位当地的森林管理员以及伤害了多名管理员后神秘失踪。如今，他终于被逮捕归案，关押在美国高度戒严的精神病监狱。 　　年轻的精神病专家卡尔特（小库珀•古丁 Cuba Gooding 饰）意图揭开鲍威尔精神失常之谜而一举成名，于是他深入到监狱直接与鲍威尔进行对话。鲍威尔一开始对这位访客充满敌意，身为人类学家的他在卡尔特面前一点也不落下风。随着对话的逐渐深入，卡尔特渐渐了解到了事件真相：鲍威尔杀害的管理员其实是偷猎大猩猩的恶棍……Nearly two years after having gone amiss in Africa, renowned anthropologist Dr. Ethan Powell is caught committing a crime and subsequently imprisoned in a Florida mental institution, where aspiring psychiatrist Dr. Theo Calder takes over his important case. Dr. Powell, who has been with a group of gorillas during all that time, is not talking at all and seems to be living in a dreamworld. Very slowly, Dr. Calder manages to reach Ethan Powell and starts finding out why Ethan killed two of the poachers. Yet Theo's case is not just about why the murders have happened, but also about how Dr. Powell became the being he is in the first place. With Ethan's silence broken, Theo is introduced into a world beyond common human comprehension: The true nature of being. He learns that mankind's control of everything is a mere illusion and that the true values of existence can't be found so easily. Ethan changes Theo's view of things forever.