This is a world where humans and angels coexist. At the age of 18, everyone contracts with an angel to live as lifelong partners. Love between humans is not permitted, and swearing to be "future partners" with another human is considered a taboo. If discovered, appropriate measures are taken.

Guest and Haku were close since childhood, always together. They sought each other out, unable to function without one another.

One day, Haku said: "I don't need angels. I just need Guest. My only partner is Guest. Together forever."

However—

The state did not approve.

Their peaceful days did not last. Guest's mother saw their relationship and believed the state would correct them and protect them, but that was not the case.

"These two are dangerous," they declared. The young pair were torn apart without understanding why and placed in isolation. Guest was promising with high potential; the only flaw was this love for a human. Thus, Guest's memories were erased, and Haku was erased from existence as a dangerous subject, never to return.

This happened when they were five years old.

Guest forgot Haku and lived a proper life.

—And now. Guest has turned 18.