This review contains spoilers.
When this episode started out, I almost have it down as a filler episode, average at best. But thankfully the twist later on turns it into a much better instalment.
This episode focuses much more on the character of Mr. Scott, who is often only used in a secondary capacity. It is good to see a story concentrate on a different crewmember from Kirk / Spock / McCoy for a change.
The late James Doohan gives a good performance as Scotty, a character who wouldn't ordinarily hurt anyone, but just cannot explain his whereabouts during the terrible murders.
The first half episode plays as a whodunit, and is well plotted, though does very on being over-talky in places.
But it is the revelation about the true identity of the killer in the last fifteen minutes or so that really brings the episode to life. The revelation that the murderer is actually Jack the Ripper, an alien, is a real shock, and plays out really well.
The scenes where Redjac (the Ripper) takes over the Enterprise computer and jumps from body to body are great. The voice work of Redjac in the computer is well done, and actually sounds genuinely scary.
All-in-all, the episode ends up much better than it looks to be at the start, with some very good moments.