Time and Again (Star Trek: Voyager)

Plot

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. Signs of temporal anomalies have been left in its wake. Janeway and Paris become caught in one, finding themselves on the planet the day 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 saboteurs threatening to compromise one of the polaric power plants. Janeway and Paris's strange Starfleet equipment prompt the saboteurs to believe they are infiltrators, so they confiscate the equipment, bring forward their sabotage plan and force Janeway and Paris to accompany them to the power plant.

Meanwhile, a day in the future, Kes's nascent psychic ability allows her to identify that Janeway and Paris have fallen back into the past. The remaining senior Voyager officers develop a method to create a short-lived rift to the past through which they hope to evacuate Janeway and Paris.

The saboteurs use Janeway and Paris as a diversion to allow them access to the polaric plant, during which Paris is shot and wounded. As they begin their sabotage, the Voyager crew initiates the rift. Janeway recognizes that it is the rift which, if not closed, will trigger the detonation that kills all life on the planet. The saboteurs allow Janeway to use her phaser to force the rift to close, changing the future.

Events then 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. In accordance with the Prime Directive, Voyager refrains from communicating with it and continues on its journey home.