The Game in Four Hands

By the age of sixty, he had swallowed numerous books about all sorts of time travelers, but when he once horrifyingly realized that he was in the body of his 15-year-old self in 1978, it turned out that the writers... had shamelessly lied.
First, he has to negotiate with his teenage self, learning to coexist together. And then, memory presented an unpleasant surprise: it turns out that it hides the last, most important years lived there, in the future, from its rightful owner. And it is precisely in those years that the key to averting such a disaster from the country, from his loved ones lies, that all misfortunes are lesser sorrows… Moreover, the time for acclimatization and building connections for the time traveler is short. His fate is that of a sprinter, not a marathon runner. And to access the hidden corners of his own memory, he can only do so by establishing interaction with his fifteen-year-old self. Thank goodness there is a hint available—in one favorite book from childhood…
What a task, huh? And who, actually, promised that it would be easy?