A 29-year-old female was admitted under section 2 of the mental health act with an acute psychosis. Her condition deteriorated as she developed a fever and worsening hallucinations. Brain imaging was normal but a lumbar puncture revealed a reactive cerebrospinal fluid. A transvaginal ultrasound revealed a left ovarian mass suggestive of a teratoma. A diagnosis of anti-N-methyl-D-aspartic acid receptor (NMDAR) limbic encephalitis was made but the anti-NMDAR antibody was found to be negative. The patient proceeded to a laparoscopic oopherectomy and a 5-day course of plasma exchange. The patient has made a gradual improvement and the anti-NMDAR antibody has since become positive.