After the death of Michiko Kawabata, Mei Kashiyama began to suffer from anxiety, hallucinations, and fainting spells, as if someone else is in her head. Tatsuhiko Emaya met Mei when he saved her from a fall after she fainted. Through hypnosis by Emaya's uncle, a psychologist, she learns about the cause of her mental illness, and breaks free with Emaya's help.

(Source: Mangapedia, translated)

Note: Includes the additional chapter, M no Kaihou.