That's right. The Enterprise's computer is taken over by an advanced, non-corporeal alien being responsible for the historical murders of Jack the Ripper. Some may consider this a "spoiler", as the ultimate reveal comes as the second-act plot point launching into act three. Here I will attempt to explain that voicing the ludicrous concept at this episode's heart spoils it no more than it spoils itself.
There can be an innocent, whimsical joy about many of the less-than-serious Star Trek installments. Here, any joys to be taken are undercut either by expositionally waterlogged techo-babble or its direct opposite: situationally inappropriate stabs at humor. It's as if the mediated approach was thrown out the window, taking itself simultaneously too seriously and not seriously enough.
With an added element of retrospective hilarity, the man first possessed by the entity (and, inexplicably, the voice of the entity post-possession) is played by the man who voiced Piglet in Winnie the Pooh. So, now, here is what the modern analyst (or even casual viewer) is left with: An exposition-heavy depiction of Jack the Ripper -- an alien that sounds like Piglet -- possessing a spaceship after framing a Scotsman for the murder of women, targeted only because it has an appetite for fear (And "women are more easily and more deeply terrified," says Spock.) causing the crew to dope themselves up with a happy hypo-spray so that there will be no fear for it to feed on.
Thus is described what is perhaps the most absurd premise for an episode yet encountered. What should have been a joyously campy romp betrays itself with an extensive and vain attempt to legitimately ground itself as a science-fiction concept. Its execution robs the modern viewer of even the sarcastic appreciation afforded with other conceits nearly as outrageously presented in Star Trek's muddled logbook.
It will surprise no one to learn that the teleplay for this episode was adapted by the author of an entirely unrelated short story published in 1943. It has received criticism for its misogynistic sexism, but this artifact of the times pales in comparison to its true crime: it is one of the worst episodes of one of the best television shows of all time. An embarrassing stain on the hull of the Enterprise.