Catboy and An Yu are walking along the bamboo forest at Mystery Mountain. Since An Yu offered to show Catboy around the place, he suggests that they should go to the top of the mountain to practice some martial arts moves in the sunrise. Before they can do that, An Yu says that they must first hear the bamboo forest. They meditate there, but only for a few seconds.
Catboy and An Yu climb up to the top of the mountain, and once they get there, before they can open the doors, An Yu says that they must feel the mystical powers flowing through the walls. To her surprise, though, the doors open, and inside the gates are the other PJ Masks members with a surprise birthday party for her.
An Yu is confused by what is going on in front of her, as she does not know what a party is. The PJ Masks explain to her that a party is where friends get together to celebrate the celebrant, which in this case is An Yu herself, with dancing moves and a big birthday cake. However, to the group's surprise, one of the dragon statues grabs the cake. Nevertheless, the PJ Masks decide to dance instead, and the music starts. An Yu, enjoying this event, joins them, and everyone dances together for the rest of the night.
However, while he has visited Mystery Mountain before in the episodes "Race Up Mystery Mountain" and "The Mountain Prisoner", it is currently unknown how he properly met her as they seem to have met before.