Li Shiyi, the leader of the Northern sect of tomb raiders, along with her companion Little Tu, found a baby girl during their expedition. The coffin contained a single character "Jiu," and so they named the girl Song Shijiu and took her with them, and that's the start of this story. She didn't know that their relationship extend back to a time long ago… You knock on a coffin three times. Once to ask where it's from, once to ask where it's going, and a third time to ask if it would like to stop and have a drink before leaving? (Source: Bilibili, translated)