In Norse mythology there is a huge ash tree - the Yugatrasi cosmic tree. It runs through nine worlds and is divided into three levels.
The upper level is: Elfheim, the country of elves, Asa Garden, the country of gods, and Mosbiheim, the country of fire; The middle level is: Atrium, the country of humans, Jotunheim, the country of giants, and Vanaheim, the home of the Vanir; The lower level is: Severnheim, the country of dwarfs, Hela's country of death, and Niflheim, the ice world.
Gu Shuyu, who was exploring the unknown jungle with his best friend, has been separated from his best friend for a day and a half...