A supreme example of this show and whats more its just the first of a two parter. A very good opening does the job of setting the scene perfectly as the mystery of finding Data's head deepens when he makes first contact with the aliens. The execellent use of his commentary allows us to get a stronger sense of helplessness as we dont even see what happens. It just enhances the situation when the team finally go after him and see what he saw when he disappeared.
In the meantime, the scenes, settings and supporting character do a good job of showing up what 19th cen San Fran could have felt like and set up the turn of events that are to come - excellently.
One particular scene with Marc Aliamo in a poker game has some excellent dialogue. The bellboy is probably the best supporting character though, with the way he takes advantage of Data, but still helps him, very "our time" humanity.
The villians in this episode already made threatening by our lack of knowledge, their supposed use of time travel and their etheral nature, are made even worse when we see one example of their "crime" against a human and their return to the "nest". The whole thing isnt natural and will make you want them to be defeated more than the ordinary villian.
The only real downer is the way in which we are expected to accept the way Data integrates into 19th Century life. I guess thats probably too much to ask, but there you go!
One of the best episodes with a bit of everything. Worth the watch.