Voyager's sensors detect a polaric detonation from a nearby planet. On arrival, they find the planet's population has been completely wiped out. An away team, including Captain Kathryn Janeway and Lieutenant Tom Paris, transport to the surface, and estimate from the ruins that the explosion only happened a day earlier, and signs of temporal anomalies were left in its wake. Janeway and Paris become caught in one, traveling to a day in the past, before the explosion. Quickly integrating themselves into the general population, they learn that the population is powered by polaric energy, an option that has been met with some protest. Janeway and Paris become caught up with a group of terrorists threatening to destroy one of the polaric power plants. The terrorists question their strange Starfleet equipment, believing them to be used to infiltrate the group, and forces them to leave the equipment behind as they go off to a nearby plant.
Meanwhile, a day in the future, Kes' nascent psychic ability allows her to identify that Janeway and Paris have fallen back into the past, and believes they will be caught in the polaric detonation. The Voyager discovers a method to create a short-lived rift to the past, through which they can evacuate their fellow crewmembers.
The terrorists, along with Janeway, stage a protest to allow them access to the polaric plant. As the terrorists begin their sabotage, the Voyager crew initiates the rift, appearing before Janeway. Janeway recognizes that the rift, if not closed, will interact with the polaric energy, creating the detonation that kills all life on the planet. Janeway uses her phaser to force the rift to close, changing the future.
Events return to the start of the episode; Voyager detects the nearby planet, bustling with a pre-warp civilization using polaric energy. Kes appears on the bridge, concerned about a feeling of deja vu, but is relieved to see the planet's civilization is alive and well. Under the Prime Directive, the crew is unable to communicate with it, and Voyager continues on its journey home.