Relativity just says "nothing about space seems to require a preferred reference frame", not "such a thing as a preferred reference frame can't possibly exist".

Relativity just says "nothing about space seems to require a preferred reference frame", not "such a thing as a preferred reference frame can't possibly exist". If we're allowing for the discovery of time travel in the story, I'm willing to allow for such a discovery as well. In reality I'd bet neither are realistic, but that's what makes the stories interesting.