From MangaHelpers:
One day, Serizawa fell asleep at work and had a dream about a past love that he wronged when he was in middle school. On the way home, he was involved in an accident, and a woman who knew him called for an ambulance. Now the woman has his phone number, but he doesn't know her identity. Serizawa thinks back over the women he has loved in the past, trying to determine who this woman that has re-entered his life is.