Time Traveler's Disease.

On the 18th of January, Meguru and Mirai's wedding anniversary, Meguru falls down the emergency stairs at her workplace and dies. Mirai, overwhelmed by sadness, develops a mysterious illness as a result, which is triggered due to fluctuations in his emotions. Before he knew it, he had returned to the past to the time she was still alive. As he relives his life from this point, he discovers a side of Meguru he previously wasn't aware of, as well as relationships he had no idea about. Meanwhile, he starts to search for the real cause of Meguru's death and takes action in order to save her.