Mystery of Bizarre Deaths in Girls' Dormitory

Why does that very familiar person always appear every time I go to a museum, as if I have met him somewhere, and seems to have had a relationship with him? Did we really come back from that era together? Why does his girlfriend look the same as me? Where is my sister? Where should I start to find this series of questions? What is my relationship with that young lady from the Qing Dynasty... This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.