Anastasia Club

The main heroine of this history is Tsubaki Kamui, the granddaughter of a noted antiquarian. One day, in her shop comes a mysterious man, Sena Takuto. He is trying to find proof that he is indeed the grandson of the last surviving member of the Russian Empire, Anastasia. In his possession is something he wants Kamui's grandfather to see and appraise to prove his claim. However it seems like something of great importance to the late Anastasia was stolen recently. Does Takuto have anything to do with this? Will he succeed in his quest to prove his lineage? And what does Kamui have to do with this? Why?