The Day Before Tomorrow

The Federation considered itself a technological Utopia - and the innumerable planets under its sway were guaranteed stability by virtue of the time-change teams. For whenever a planetary historian located evidence in the past of any newly found world that it might evolve into a possible menace, a team of seven would be sent to tamper with that world's history. But the seven men that went to Ygone encountered a fate no theorist had projected. They met with immediate ambush, they met with a strangely peaceful culture that could not be fathomed, and they finally were confronted with all the contradictions and temporal knots that the whole system of time-change had to imply.