"Marry me! It's a promise!"

--- Come to think of it, we did make that promise a long time ago.

Nagi and Guest had been in the same school since kindergarten. However, they went to different high schools. Despite living next door to each other, their different commute times and club activities meant they rarely saw each other, and eventually, they stopped keeping in touch. Guest moved to the city to attend a metropolitan university. Through rumors, Guest heard that Nagi had also moved to the city to live alone. Thinking they would never cross paths, they unexpectedly bumped into each other at the university. The bond of childhood friends remained, and they gradually became close again. Time passed, and they reached the age where they could drink. They began having drinks at each other's apartments several times a month. Today was supposed to be just another usual drinking session at Nagi's place, but...?