"Scientific plausibility" can be tempered by exoticism, time, and distance. It can also be flatly ignored because it's just a plot convenience, or substituted as a metaphor for the real story you are telling.
"This technology is so exotic that we barely understand it ourselves!"
The technology's narrative role is that it pushes credibility, or is difficult to grasp for the characters. Used as a positive it conveys a sense of wonder. Science is always on the edge of new discoveries that will transform our lives.
Used negatively, the exoticism implies a threat. Scientists are playing god. Technology is going too fast and humans should slow down. This is ultimately a conservative message, a fearful message. Man should not attempt to jump the line, or the unseen hand of cosmic justice will slap them down.
"In the final decade of the 21st century, men and women in rocket ships landed on the moon."
The quote above is from Forbidden Planet (1956). The first actual moon landing would be only 13 years later, not 140 years – maybe it's the "and women" part that sounds too far fetched so they had to set it further in the future. Too many innovations in a short amount of time will jeopardize suspension of disbelief (having too many "buys" in the story). 1956 audiences could not accept both rockets and women on the moon in just 13 years.
Add another century and… well, maybe.
By 1966, Americans could accept that body swapping technology exists in the 23rd century, but not female Starfleet captains. No technology is too outlandish given enough time, another century, another millennium (so long as the people with lady-parts are kept away from the things that go boom).
"...the end of the universe? Or do you see this as the beginning…?"
The above quote is from another Star Trek episode where they accidentally travel so far away that the laws of physics are different. Matter and thought become blurred in metaphysical soup. Strangeness increases with distance.
By extrapolation, objects from far away bring their exotic properties with them, hence signals from a distant galaxy, artifacts recovered from a distant planet, even the traveling strangers themselves can show up on our door bringing a sophisticated technology that requires no research or development phase. The general public can have sudden (unearned) access before it is fully understood.
In Altered Carbon the cortical stacks come from technology discovered on an alien planet (you'd think the discovery of aliens would factor into the story somehow, but nope, just a pretext for instant technology).
I'll separate the last two because they are strictly narrative, and ignore any worldbuilding pretext.
It's perfectly valid to leave it unexplained. If it doesn't serve the narrative, don't waste time on it. We have a very unflattering word for this: infodump. If the climax is ultimately about shooting a gun at the thing, or rescuing a princess, or doing a distraction dance, no amount of technobabble "filler" is required to explain it.
The final narrative function is that it's a metaphor. The "mystery" is only explained through subtext and theme, but not directly through facts and science. That impenetrable barrier at the edge of space is really a metaphor for the limits of human understanding. When it mostly makes sense within the context of the storyworld, or if it mirrors a character's internal state, it's probably more symbolism than metaphor (the frozen Oort Cloud of the soul). Metaphor is something completely outside the storyworld. The psychic planet Solaris is a metaphor for depression (isolation, guilt, mental health), since the story doesn't feel the need to explain it in a literal sense the author is free to explore the metaphor more completely.
Horror can be considered a metaphor since the story logic will follow the theme (haunted house in spaaaace) even while characters comment on events that are unnatural or unexplainable.