Epithelial ovarian cancer (EOC) has one of the worst prognoses among gynecologic cancers. An appropriate screening method is not available for EOC, and the initial symptoms such as abdominal pain or bloating, difficulty in eating, and urinary urgency are vague. As a result, most cases of EOC are diagnosed in an advanced stage of disease. We report novel insights gained from the case of a 45-year-old, gravida 0, para 0 woman who presented to the emergency department with complaints of general weakness, fatigue, and myalgia over the previous 2 months. She reported progressive muscle weakness of the upper and lower extremities, leading to difficulty in walking. Serum muscle enzymes, such as creatine phosphokinase, were markedly elevated. No evidence of malignancy was detected on an imaging study. A biopsy of the left vastus medialis muscle was performed. The results were consistent with those of primary myopathy with myofibrillar disarray, suggesting paraneoplastic necrotizing myopathy. Explorative laparotomy was performed to evaluate the results. Histopathological analysis of the full specimen revealed a grade 3 ovarian serous adenocarcinoma with direct invasion to the rectum. The possibility of EOC should be considered in unknown primary malignancies with initial complaints of muscle weakness or myalgia.