The Exchange Letter 2016 on netflix

Mariko Okano’s boyfriend Junichi Nagata leaves suddenly to become a volunteer teacher in a remote country. Mariko and Junichi’s families are both left wondering why he departed so quickly, without discussing his plans with them first. The next day, a woman’s charred body is discovered in a warehouse. The woman turns out to be Ayako Oshima, the mother of a student who was suspected of arson in a warehouse blaze 15 years ago. Oddly, Mariko had been trapped in that warehouse fire with another student, but was saved by Junichi. However, the other student died in the blaze. Soon after, Mariko receives a letter from Junichi, in which he confesses to a crime Mariko was not aware of. Are the two fires related? And what was the crime Junichi confessed to?