Threshold (Star Trek: Voyager)

Threshold is a second season episode of Star Trek: Voyager.

Plot
The Voyager crew discover a special type of dilithium crystal that allows ships to travel at Warp 10, thereby travelling at infinite speeds. When Tom Paris comes back from a test run, however, he suddenly starts mutating into an amphibian-like creature.

Why It Can’t Hold There

 * 1) The story revolves around people getting transformed into amphibians by travelling too fast. Seriously, do we even need to give any other reasons as to why this is a terrible episode? (We will anyway, because there's lots of them)
 * 2) The ability to travel at Warp 10, something that Captain Janeway even explicitly calls the biggest breakthrough since faster-than-light travel was discovered three centuries prior in-universe, is not only never mentioned again after this episode, it's largely thrown to one side less than halfway through the episode.
 * 3) On a similar note, the explanation of why they can travel at Warp 10 is completely nonsensical. It's like thinking you can drive your SUV faster-than-light by using the right type of gasoline. There's also no explanation as to why they can't use the special dilithium to just increase the speed of their normal warp drive to the point where the journey home would be manageable in a few years, rather than travelling at infinite speed.
 * 4) What happens to Tom (and later Janeway) has an even stupider explanation, namely that this is the way that humanity is destined to evolve, and travelling at infinite speed accelerated the process massively. To those who actually know what the theory of evolution is, this is offensively stupid, and to those who believe in young-earth creationism or intelligent design, it's just plain offensive.
 * 5) Part of Tom's mutation involves him suddenly coughing up his tongue, which is either gross or just stupid depending on the viewer.
 * 6) The Doctor explains very clearly that they have to cure Tom as soon as possible, otherwise he'll be too badly mutated to restore... and later on manages to cure an even more mutated, and no longer even humanoid, Tom and Janeway with no trouble.
 * 7) Even with the complication of people mutating after travelling at infinite speed, they don't explain why they don't just use the method to instantly bring Voyager home and then employ their cure, since we see that even people in advanced states of mutation can be cured, and the Doctor is a hologram and therefore immune to being mutated.
 * 8) The scene where the mutated Tom escapes from his shackles in engineering happens completely off-screen.
 * 9) The traitorous Michael Jonas is completely wasted in this episode, only appearing for one brief scene which has no relevance to the plot.
 * 10) Neelix is as annoying and pushy as in most other early Voyager episodes. He also manages to solve a problem that initially made the Warp 10 flight impossible, thereby being indirectly to blame for the episode's events happening at all.
 * 11) Future episodes of the series explicitly deny that the events of this episode ever took place. For perspective, even Star Trek V: The Final Frontier, which Gene Roddenberry himself personally decanonized and future Trek writers went out of their way to avoid referencing, was never outright contradicted in this way, and was eventually acknowledged in Star Trek Beyond, 27 years later.

Redeeming Qualities

 * 1) The make-up effects used to depict Tom's mutation are actually pretty good for a TV show, and won an Emmy award.
 * 2) A couple of good lines from the holographic Doctor.
 * 3) Robert Duncan McNeill somehow gives a decent performance despite the ridiculous situation and dialogue he's given.

Reception
Several online reviewers, including SFDebris and The Agony Booth, have named this as the worst episode of Voyager. Fans at the Star Trek 50th Anniversary convention agreed, naming it as the worst episode of the series, and the third-worst episode of Star Trek in general (behind "Code of Honor" and "These Are The Voyages...").