Plot summary
After a bat'leth tournament, Lieutenant Worf returns to the Enterprise and a surprise birthday party. He starts noticing subtle changes; the flavor of the cake changes, and a painting given to him by Data appears on a different wall. The changes become more pronounced; as a Cardassian vessel attacks the Enterprise, unfamiliar controls result in Worf failing to raise the shields, leading to the death of Geordi La Forge. Despite retaining his memories, Worf has no evidence that reality has changed, and his logs support the stories given by the other crew members. Other major changes occur: Riker is captain of the Enterprise in a reality wherein Picard was killed by the Borg, the Bajorans became the oppressors of the Cardassians and enemies of the Federation, Wesley Crusher is a lieutenant on the Enterprise, and Worf is married to Deanna Troi.
The crew find that Worf's RNA has an unusual quantum signature; he is from a different universe. The crew theorizes that Worf, during his return from the bat'leth tournament, passed through a time-space fissure; the shuttlecraft's engines and later proximity to La Forge's VISOR caused Worf to shift between universes. Each universe covers a different possibility; anything that could happen has done so in a parallel universe, hence the changes in the reality that Worf is experiencing. The Enterprise returns to the location of the fissure, attempting to return Worf to his original universe, but is attacked by a Bajoran ship, which causes the fissure to destabilize and the various realities to merge. Innumerable Enterprise s begin appearing at an exponential rate.
Data determines that the only way to restore the realities is to send Worf via shuttlecraft to the Enterprise of his universe, passing through the fissure and using the shuttlecraft's engines to reseal it. Locating the correct ship, Worf begins travelling back but is attacked by an Enterprise from a Borg-overrun universe, its crew refusing to return. The Enterprise that he left fires upon the ship, accidentally destroying it. Worf passes through the fissure, returning to his universe with a single Enterprise in front of him. Worf finds that no time has passed since he initially entered the fissure. Expecting a surprise party, he finds only Troi waiting to give him a present and, knowing that the two are married in many alternate universes, he invites her to share dinner with him.
Credits
Adapted from Wikipedia contributors, Parallels (Star Trek: The Next Generation). Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.