A mysterious old man stops Paris-chan, the director of the Girls' Detective Agency, and entrusts her with an ancient ring that is being targeted by a phantom thief known as the Jewel Demon. However, when Paris-chan is left alone, the Jewel Demon appears and attacks her, trying to steal the ring by force! Paris-chan is saved by a boy named Prince Kari, and together with her fellow Girls' Detective Agency colleagues, they try to capture the Jewel Demon, but...