This article contains spoilers for the penultimate episode of "Star Trek: Lower Decks."
The penultimate episodes of "Star Trek: Lower Decks" -- season 5's "Fission Quest" -- takes place away from the U.S.S. Cerritos on a mysterious, covert interdimensional vessel called the U.S.S. Anaximander. The Anaximander is commanded by Captain William Boimler (Jack Quaid), a transporter duplicate of Brad Boimler, created several years ago on the series. Captain Boimler belongs to the ultra-secret organization Section 31, and he is on a mission to repair fissures in the spacetime continuum. It seems someone has been ripping holes in space, allowing free access between dimensions. This is causing chaos, as alternate versions of familiar "Star Trek" characters are haphazardly cross-pollinating, invading each other's universes with accidental frequency. Boimler and his crew use the tools on the Anaximander to seal up holes and find the hole-ripping culprit.
The multiverse came into play in the first episode of "Lower Decks" season 5, which saw the crew of the Cerritos come face-to-face with parallel universe versions of themselves. Thanks to recent hit films featuring Spider-Man, Deadpool, or Michelle Yeoh (or less successful ones featuring Morbius or The Flash), mainstream audiences are likely familiar with the multiverse concept by now. There are an infinite number of universes and, as such, an infinite number of you. Any alternate version of you exists on another plane. "Fission Quest" at least has several lines of dialogue satirizing how tired multiverse characters can be.
Captain Boimler's crew, to exploit the concept to its fullest, is made up entirely of parallel universe characters, each one familiar to deep-cut Trekkies who have been following the franchise for decades. Let's look at each character and see where they may have been plucked out of "Star Trek" canon.