After competing in a bat'leth tournament on Forcas III, Lieutenant Worf returns to the Enterprise and is treated to a surprise birthday party, much to his annoyance. As he attends the party, he starts to notice subtle changes, such as the flavor of the cake changing, and a painting given to him by Data appearing on a different wall. The changes start to become more pronounced, and Worf's confusion leads to the Enterprise being damaged by a Cardassian attack which results in the death of Geordi La Forge. Despite retaining his initial memories, Worf has no evidence that reality has changed and his own personal logs support the stories given by the other crew members. Other major changes occur: William T. Riker is seen as Captain of the Enterprise in a reality wherein Jean-Luc Picard was killed in The Best of Both Worlds, the Bajorans become the suppressors of the Cardassians and enemies of the Federation, Wesley Crusher is a lieutenant on the Enterprise, Alyssa Ogawa is the Chief Medical Officer, and Worf finds himself married to Deanna Troi. Upon investigation, the crew find that Worf's RNA has an unusual quantum signature and that he is essentially a Worf from a different universe. The crew theorizes that Worf, on his original return from the bat'leth tournament, passed through a time-space fissure, with the shuttlecraft's engines causing Worf to quantum flux between several parallel universes. Data determines that Worf's proximity to La Forge's VISOR was causing Worf to shift between universes, and that each universe covers a different possibility. In effect, anything that could possibly happen has done so in a parallel universe, hence the believable changes in reality that Worf was experiencing.
The Enterprise returns to the location of the fissure, attempting to return Worf to his original universe. The Enterprise is then attacked by a Bajoran ship, which causes the fissure to destabilize and the various realities to merge, resulting in over 285,000 Enterprises appearing in the same area of space. 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. After locating the correct ship, Worf begins to travel back but is attacked by another Enterprise that has come from a Borg-overrun universe, its crew refusing to return. The Enterprise that he left fires upon the hostile ship and destroys it. Worf safely passes through the fissure, finding himself back in normality with a single Enterprise in front of him. After boarding, Worf finds that no time has passed since he initially entered the fissure. When he returns to his room expecting a surprise party, he finds only Troi waiting to give him a single present and, knowing that the two are married in many alternate universes, he invites her to share dinner with him.
This episode's simultaneous portrayal of numerous different universes was cited by Roberto Orci, one of the screenwriters for the 2009 film Star Trek, as proof that, despite the movie's creation of a secondary timeline, the "regular" Trek universe was unaffected and everything from The Original Series onward still went on as it had before.