The 13th Footprint

Year 2018, Chiba Prefecture, Japan.
Momoya works as a teacher at grade school. He lives with his wife, who was a childhood sweetheart, and his son, Ao. Ao has just come back from his long days in a hospital.
They are a happy family, living in a house they built for themselves.
Momoya loves his life, peaceful days...but one day, he starts to receive strange postcards.
Each postcard tells Momoya of what happens in the future...and from the second postcard, the sender asks Momoya to save someone. Momoya can't ignore the message and runs to the place written on the postcard. It really happens...are the postcards sent from future?
He doesn't know who the sender is...and what the sender wants.
Momoya must face the "past" to protect his family.

(Source: BookWalker)