RGB.Childhood friends Ran, Ko, and Shu were born and raised in the 24-ku (24th Ward). They hung out together and were referred to a "RGB" because of their different hair color. One day, their school was engulfed in flames, and Shu tried to save Koki's sister Asumi from the inferno, but she was fatally injured when she shielded her friend Kozue with her body and died in his arms. A year later, the three friends to meet again at the memorial ceremony of the incident and afterwards their phones ring simultaneously. The phone call is from their supposedly dead friend, Asumi, which presents them with a vision of the soon to be launched Kizuna Train full of passengers heading towards their friend Mari holding her puppy on the tracks, presenting them with the Trolley problem...