We report the case of a young male who gradually developed a subacute demyelinating polyradiculoneuropathy. Routine examination and extensive biochemical investigations revealed demyelinating neuropathy, M-Band in electrophoresis. The patient referred for the whole-body 18F-fluorodeoxyglucose-positron emission tomography/computed tomography to look for skeletal and bone marrow lesions. The patient was found to have rib plasmacytoma. The case is rare because of infrequent association with the site, age, and symptoms.