A time-ripple sends Janeway and Paris to the past of a doomed planet.
Janeway and Paris are caught in a temporal event while on an away mission to a planet, and they find themselves thrown back a single day, just before a devastating disaster destroys a civilization. Racing against time, they try to learn what caused the catastrophe and whether they can avoid it without damaging the timeline.
It is one of the series' early time-travel stories, and it sets down the show's habit of letting the crew's own technology become part of the problem. The Prime Directive is a constant worry for Janeway as she weighs how much she can interfere in a world that is not her own.