Breast cancer affects about one in eight women over the course of her lifetime. Occult breast cancer, in which primary breast cancer is detected without evidence of disease in the breast itself, comprises up to 1% of new diagnoses; this is typically detected from abnormal axillary lymph nodes, and distant metastases are rare. Here, we present an unusual case of occult breast cancer presenting as upper extremity pain, edema, and weakness, with a metastatic mass to the brachial plexus being the only site of disease.