Introduction: Neuroendocrine neoplasms (NEN) are typically characterized by an indolent and slow-growing nature. Due to their gradual progression, these tumors are often detected incidentally, presenting as metastatic deposits rather than primary masses. Neuroendocrine tumors (NET) with an unknown primary origin are relatively common in clinical practice, accounting for approximately 12–22% of NEN cases. In such instances, biopsy plays a crucial role, enabling pathologists to determine the tumor type and, where possible, the site of the primary tumor through histopathological analysis. The classification of NETs is based on their histological differentiation and grading. Low-grade, well-differentiated tumors tend to follow a more indolent course, while high-grade, poorly differentiated neoplasms exhibit rapid growth and aggressive behavior. This classification is closely linked to the clinical presentation and prognostic outcomes of the patients. Presentation of the Case: A 74-year-old female hospitalized at the internal medicine service due to difficulty in breathing during minor physical exertion, thoracic-abdominal discomfort, leg edema, after exclusion of pulmonary thromboembolism with pulmonary angio-CT and acute coronary syndrome in the emergency department. Abdominal ultrasound raised the suspicion of a cholecystic tumor with multiple hepatic metastases. Subsequent examinations with contrast CT and MRI abdomen, EGD, and colonoscopy resulted in multiple secondary hepatic lesions and cholecystic calculi. CT-guided biopsy and immunohistochemical analysis of the liver mass identified poorly differentiated small cell carcinoma with positive staining for PanCK, Chromogranin, CD56, and a Ki67 proliferation index of 70%. After extensive examinations were unable to determine the primary origin of the tumor, oncological assessment recommended chemotherapy with cisplatin and etoposide for the treatment of stage IV small cell neuroendocrine tumor of unknown primary origin with liver metastasis. Considering the advanced stage of the disease, the patient opted for palliative care. Conclusion: In cases of neuroendocrine tumors with well-differentiated cells, more detailed diagnostic examinations are performed to find the primary site, to perform surgical intervention, or to start systemic therapy depending on location. The location of the primary site is also important in determining the prognosis of neoplasia. In the case of poorly differentiated neoplasms, detailed diagnostic investigations to identify the primary source do not affect the prognosis of the disease.