Many people when reading the summary for this drama may find the concept silly, but the show uses it masterfully to show how different people cope with death.
The biggest thing in the drama is forgetting about someone when they're gone, how many people will forget you, but there will always be at least one person who will never forget. By the end, Mitsuru is unable to forget Koichi and is unable to move on, forever holding on to a lingering sadness.
One of my favourite messages from this series, said by Mitsuru's father, is that you don't have to force yourself to heal. It's always good to try to move on, to heal, but sometimes, a lot of times realistically, it's impossible and that's okay.
This series has become one of my favourites and is one of the very few dramas that has actually made me cry. I'm a sucker when it comes to death metaphors in shows, so this one really hits the spot. This is a short review, but I don't have much to say besides you should really watch this drama, for its message is one many people need to see and hear.