VOY S4E08 - Year of Hell, Part I

Voyager enters a region of space controlled by the Krenim, who at first seem to be a small, easily avoided regional power. Before the viewers eyes however, they are transformed to an advanced and belligerent nation and Voyager constantly faces destruction. The cause of this transformation is a Krenim scientist whose obsession with restoring his people and his family to him has let him to manipulate time itself.