After the body dies, the soul lives on. Many spirits progress toward the world of light, through the levels of the spirit world where they study how to improve their souls. These spirits are sometimes reincarnated into the physical world, and sometimes they are sent back as guardian spirits for other living people. Guardian spirits must protect their charges souls from the blackness that oozes up from the world of darkness below.
Riyon is a very laid-back spirit, who often skips her spirit classes to fool around. To train her further, she is sent back to the physical world as a guardian spirit. Instead of following a human from birth, she is helping a beleaguered man, the Japanese Prime Minister Kasuga Soichiro, in the last year and a half of his life. After an unexpected close call with death, Kasuga gains some unusual talents, including the ability to see Riyon. Will Riyon and Kasuga together be able to protect his battered soul?
(Source: MangaHelpers)
— Description from AniList community (anilist.co).
