Title: Oufuku Shokan - Jugo nen Go no Hoshu
Japanese: 往復書簡～十五年後の補習
English meaning: The Exchange Letter
Episodes: 1 (Tanpatsu)
Duration: 01h:38m:54s
First broadcast date: September 30, 2016

Plot (from AsianWiki): Mariko Okano (Nao Matsushita) works as an OL and dates Junichi Nagata (Hayato Ichihara). Suddenly, Junichi leaves Japan. He applied to become an international volunteer and took a job to teach kids in a developing country. Junichi is not coming back for 2 years.
Mariko feels uneasy with his situation, because he did not talk to her about teaching overseas. She can’t reach him by phone and can now only communicate with him through letters.
Next morning, a burned female body is found at a lumber warehouse. Detective Ryuzo Kameyama (Takeshi Kaga) investigates the possible murder and arson case.

English subtitles - http://www.d-addicts.com/forums/viewtopic.php?f=13&t=164259