After being abused and kicked out of her family's estate, Tsubasa Tenjou, sought out her childhood friend, Kyojuro Rengoku for help. Years later, she climbed the rank of kinoe among the Demon slayer corp and becomes Kyojuro's fiancee as well. Tsubasa couldn't be happier... until one fateful mission turns everything upside down for the young couple. Rengoku had a fiancee all this time? What happened to them before the events of Mugen Train? Takes place between Rengoku's spinoff gaiden up until Mugen Train Arc.