The crew of Voyager detect a massive 'polaric' detonation and encounter as its source a devastated planet, with remains of a civilization, and later discover temporal anomalies on its surface during an away mission. Captain Janeway and Lieutenant Paris get caught in a time fracture on the planet and travel back in time by a single day to a bustling civilization, before the detonation has occurred.
It transpires, that the civilization uses polaric energy as an energy source. After encountering protesters who are opposed to this energy source, and being accused of infiltrating the protest group as a government spy, Janeway learns of a planned terrorist act and suspects that it is responsible for the detonation which destroys the planet.
Being held as a hostage by the group inside the polaric energy facility after a forceful invasion, an opportunity arises for Janeway to prevent the terrorists from carrying out their sabotage and she holds them at gunpoint.
Meanwhile (one day in the future), the crew of Voyager have been researching ways to rescue Janeway and Paris using the time fractures. On the advice of Kes, whose undeveloped psychic senses detect Janeway's presence in the past at a particular location on the planet, the rescue team create a temporal rift. This becomes vital to the away team after Kes says 'This is where Captain Janeway died.'
The temporal rift exits one day in the past at the same location, near Janeway, and it then transpires that it is the crew's own rescue attempt which causes the polaric detonation by forming a dangerous rift inside the polaric energy plant. Janeway uses her phaser to close the rift, at which point the timeline is destroyed and the story appears to begin again.
No such detonation is detected however, and in place of a devastated planet, a pre-warp civilization is discovered. No one except for Kes is aware of the events which took place in the alternate timeline. Only the viewers know that Kes's mysterious abilities are accurate, because at the end of the episode, after the alternate timeline is negated, the crew responds to her vision but finds nothing.