25-year-old Thomas Neville suffers from monophobia, the fear of being alone, brought about by a childhood trauma. To treat his monophobia, Thomas and his psychiatrist, Dr. Ramon Medeiros, rent a home and begin exposure therapy. When a malevolent entity latches onto Thomas, Thomas must not only convince Dr. Medeiros that the entity is real but must also persuade Dr. Medeiros to help him expel the entity.