Assassination on Theseus

The book opens a cycle of works by Kir Bulichev about the agent of the Intergalactic Police, Kora Orvat, a girl found in infancy on one of the remote planets of the Galaxy and raised on the Children's Island in... a top-secret research boarding school-shelter for lost orphans like herself.
…There is an emergency at the island shelter. Two young people, who are in love with each other, have disappeared. Is this an escape or a kidnapping? Commissioner Milodar, commander of the Interpol forces in the Northern Hemisphere of Earth, is conducting an investigation and during it meets one of the students, Kora Orvat, the friend of the missing girl. And if it weren't for her help, it is unknown what unpredictable results the actions of Interpol would have led to («Children's Island»).
…Besides our Earth, there is Earth-2, which also orbits the Sun, but… in a parallel world. And in a certain place in Crimea, there is a transition point from our planet to its strange counterpart. Who needed this window, who is its creator, and what purposes does it serve? All this is to be discovered by Kora Orvat, the best of the best agents of the Intergalactic Police («Halfway from the Cliff»).
…Could Kora have thought that one day she would become a bodyguard? And not just anyone, but the heir prince of a distant alien kingdom. Death follows Prince Gustav closely, with assassination after assassination. To escape danger, the prince, disguised as Theseus, plunges into virtual reality and hides in Ancient Greece among its mad gods and quite real monsters. Kora Orvat secretly accompanies her ward («Assassination on Theseus»).