Chakotay and Kim are on an ice planet investigating the crash site of Voyager. It is the future, and the entire ship and crew are frozen inside a glacier. Once they are beamed inside, Chakotay finds the long-dead Seven of Nine and summons the pilot of his shuttle, Tessa, to transport her aboard. Kim uses Starfleet technology to access Voyager's computer and activate the Doctor's program. When he demands to know what has happened, Chakotay tells him they are there to change history.
Flashback to the crew celebrating the completion of Voyager's quantum slipstream drive. The next day they will set a course for the Alpha Quadrant at a speed never before imagined. However, Paris finds a phase variance in the threshold that causes the slipstream to become unstable. Kim volunteers to take the Delta Flyer a few seconds ahead of Voyager in the slipstream. He can map it and send the phase variations back to the ship in time to make corrections. Janeway agrees to take the risk and assigns Chakotay to fly with Kim.
Fifteen years later, Chakotay and Kim explain to the Doctor what happened. Kim transmitted the wrong variance, forcing Janeway to make an emergency landing that killed the crew on impact. Kim and Chakotay made it back to Earth, but when Starfleet gave up their search for Voyager, they decided they had to find a way to correct their mistake. They stole a Borg temporal transmitter and the Delta Flyer from Starfleet, and now they can use Seven's Borg interplexing beacon to send a new set of phase corrections back in time to the crew. Just as Chakotay downloads Voyager's sensor logs, a Galaxy-class starship arrives in search of the thieves.
Captain Geordi La Forge of the U.S.S. Challenger hails Chakotay and tries to talk him out of altering the timeline, but Kim and the Doctor continue to work feverishly on Seven's Borg implant. Once the Doctor pinpoints her time of death, they use the temporal transmitter to send the phase corrections four minutes prior to Voyager's crash. In the past, Seven receives and inputs them, but the slipstream continues to collapse. Just outside of the Alpha Quadrant, Voyager crashes onto the ice planet.
When Kim realizes his phase corrections still didn't work, he doesn't have time to find his mistake. Tessa disengages the Delta Flyer from the Challenger's tractor beam, but the shuttle's warp core begins to breach. With only seconds to spare, Kim transmits a phase correction to Seven that will disperse the slipstream entirely, just as the Delta Flyer explodes. On Voyager, both the ship and shuttle are thrown out of the slipstream, effectively erasing the future. Later, Janeway informs Ensign Kim that a log entry was found encoded in the transmission to Seven: a message "from Harry Kim ... to Harry Kim."