Ayumu Higashino, known as the "Shadow Killer", was entrusted by Rainbow Son Riboen to train Sawada Tsunayoshi, the tenth generation successor of Vongola, but he was always entangled with that aloof cloud. "Higashino Ayumu, I will definitely bite you to death next time." Just like this, after witty fights again and again, Higashino Ayumu found that he was becoming more and more inseparable from that cloud...