Background: Nightmare is a prevalent parasomnia associated with various forms of psychological distress in both clinical and general populations. This study aimed to explain the experiences of a client regarding the effects of nightmares and effectiveness of dream analysis treatment on idiopathic nightmares. Methods: This qualitative study was conducted on a 34-year-old woman who participated in 8 sessions of dream analysis treatment. The client was diagnosed with nightmare disorder based on the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria for nightmare. The hermeneutic method was used to collect and analyze quantitative and qualitative data. This study utilized a simple method for hermeneutic analysis that requires only one person while maintaining research validity. Results: The findings of this study showed the frequency of nightmares decreased during the treatment. The client changed significantly during the treatment, and the dream analysis was responsible for this change. According to Bohart’s grid, there was 72% certainty of change in the client and 87% certainty that improvements were due to therapy. Useful therapeutic factors included analyzing the hidden meaning of nightmares, expressing emotions, self-awareness, and reducing helplessness. Conclusion: The results of the interventions showed that treatment processes made changes in the client that were unexpected and important to her and these changes would not have occurred without the treatment. This method, in addition to exposing the person to the content of the nightmare, leads to the discovery and decoding of the content of the nightmares and understanding of their message to resolve conflicts.