Nyanii begins as an egg saved by Melonpanna from destruction during the passage of Dream Suisei, a comet visiting Earth once every century. This celestial event sweeps the world, connecting countless minds in shared dreams of a strange realm. Melonpanna intervenes to protect the egg within this dreamscape. Upon waking, she discovers the egg has crossed into reality with her. It soon hatches, revealing a small kitten whom Melonpanna names Nyanii and raises.

As Melonpanna cares for Nyanii, a crucial truth surfaces: Nyanii's life force is bound to Dream Suisei. Survival demands her return to the comet; remaining on Earth means her inevitable disappearance. This looming separation wrenches Melonpanna's heart. The peril escalates dramatically when Baikinman accidentally awakens Muuma, a dangerous monster. This emergence amplifies the peril surrounding Nyanii's necessary journey back to Dream Suisei, irrevocably linking her destiny to the conflict against this newly unleashed threat.