"I'll bring Shiro to you. I promise!" Jin, the cat, made this promise to Mugi, who is deeply sad about losing her beloved dog, Shiro. Jin loves Mugi very much and, after hearing the story of the "Rainbow Bridge" from his owner Aki, he decides to take Mugi to meet Shiro at the Rainbow Bridge. Can Jin heal Mugi's sadness? With the help of nearby pigeons and squirrels, Jin's great plan is now underway.

The story centers on the melody played by the protagonist (Yoh). When his voice transcends time and space to reach Meguriboshi (another planet far away from Earth), many mysteries that were not told in the original story are revealed.