Tsuru ni Notte: Tomoko no Bouken

A young girl named Tomoko visits the Hiroshima Peace Memorial. There she has a mysterious meeting with Sadako, the little girl who was 2 years old when the U.S. dropped the bomb on her city during WWII and 11 when she died from the effects of radiation poisoning. Sadako takes Tomoko on an eye-opening journey.
(Source: AniDB)