Enabled by its newfound method of regular communications with Starfleet in the Alpha Quadrant, U.S.S. Voyager is assigned its first official mission in the Delta Quadrant, to locate the space probe, Friendship 1, launched some centuries earliers, which Starfleet believes would be near Voyager 's current position. The probe had contained technology knowledge such as antimatter reactors and communication devices, in hopes to contact other species for peaceful communications.
Using historical data and their own updated star charts, Voyager tracks the probe to a planet that is suffering from nuclear winter. An away team takes the Delta Flyer to the surface to retrieve the probe. They find pieces of the probe, but before investigating further, Paris, Neelix, and Lt. Carey are kidnapped by humanoid beings. Chakotay and Kim return to the Flyer where they are again attacked. They manage to stun the humanoid, but the shuttle is then attacked by anti-matter weapons. Chakotay orders the Flyer to return to Voyager as they cannot survive the barrage, with plans to return for the other three. Back on the ship, the Doctor treats the humanoid to consciousness. He states he is Otrin, one of the survivors of the population, and reports that when Friendship 1 landed on the planet, some of the population used the knowledge improperly, with the current nuclear winter caused by a failure in an anti-matter containment grid. Otrin warns that the three missing crewmembers are likely being held by Verin, a group of survivors that have come to despise humans believing humans created the dilemma on the planet and that Voyager is now here to conquer them. Seven of Nine offers the idea of using her nanoprobes to both cure the radiation poisoning that the population is suffering from, and to restore the atmosphere and end the nuclear winter. She tests her theory on Otrin, proving the idea viable.
Captain Janeway makes contact with Verin to negotiate. Verin demands that Voyager help to transfer the remaining population to a safe planet, a process that will take at least three years. Janeway believes this impossible, and instead offers to provide food and medicine for release of one of the hostages. Verin agrees, allowing Carey to be beamed back to the ship, but just as the transporter beam starts, Verin kills Carey. Janeway is shocked and relents to Verin's evacuation plan when he threatens to kill another hostage. Otrin realizes they may need someone besides Verin to speak for his people, and works with the Voyager crew to develop a plan. They successful use the Doctor to disguise himself as one of Verin's people, allowing them to infiltrate their base, render the other captures unconscious, and freeing Paris and Neelix. Otrin takes over leadership from Verin, explaining Voyager 's plan to restore their planet's environment.
With the Voyager crew returned and Otrin's signal, they launch photon torpedoes that will explode in the atmosphere with the nanoprobes, which will start a chain reaction to restore the planet. The chain reaction causes a great deal of seismic activity, frightening Verin into a panic and causing him to try to initiate the planetary defense systems which will render the reaction null. Otrin and his people stop him in time. After the quakes have subsided, Otrin and his people look outside onto a beautiful sunny day, the signs of nuclear winter eradicated.