Why did the girl have to lose her memory?

なぜ少女は記憶を失わなければならなかったのか？～心の科学者・成海朔の挑戦～

When the suspect of an arson case refuses to cooperate with the investigation, the detectives search his house and find a locked-up little girl. She doesn't talk and there are no clues to her identity, so assistant psychology professor Narumi Saku is asked to counsel her.