A 27-year-old Hispanic male presented with fever and acute encephalopathy. The examination revealed hyperthermia, tachycardia, diffuse flushing, lethargic state, ocular clonus, spontaneous and inducible limb clonus, and hyperreflexia, all suggestive of serotonin syndrome. He denied prescribed medications but reported taking several natural supplements, later confirmed to possess serotonergic properties. He was treated with cyproheptadine and supportive care, returning to baseline within 48 hours. However, cerebrospinal fluid showed mononuclear pleocytosis, and brain MRI revealed multifocal T2-hyperintense lesions not typical for serotonin syndrome. The patient was lost to follow-up before further workup could be conducted. Six months later, he developed focal seizures with sensory aura. MRI demonstrated resolution of prior lesions but showed new right frontal and left insular T2-hyperintensities. EEG revealed right frontal-temporal sharp waves. Concern about autoimmune/paraneoplastic encephalitis was raised, prompting a panel to be sent. CSF testing was positive for CRMP5 antibodies. Malignancy screening uncovered an anterior mediastinal lymphoma, which was resected. The patient remains functional and seizure-free two years post-treatment under neurology and hematology follow-up.