El barco de Teseo, 1

In 1989, 21 students and staff members of the Otosu Municipal Elementary School, a town in the Hokkaido region, died from poisoning. Now, Shin, the youngest son of the main suspect in the crime, travels 28 years back in time to Otosu six months before the school massacre. Since his father has always maintained his innocence, Shin comes into contact with the future victims and begins to investigate the events that will lead to the mass poisoning. Can the tragedies of the past be prevented, and thus their grim fate altered?