Enterprise 4x11 "Observer Effect"

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Like "Daedalus," the episode that immediately precedes it, "Observer Effect" is very nearly episode you can skip, except it has a modicum of relevance to longtime fans.

In this instance, the generic material that some fans will find comfortably familiar (aliens, well, observing the crew and making them uneasy) is redeemed by the fact that the aliens doing the observing are Organians, last seen in the original series episode "Errand of Mercy," famously one of the first appearances of the Klingons, establishing their competitive relationship with the Federation.

Now, on that level alone, there might be a level of residual intrigue.  But on another level, it was somewhat clever for the series to do an episode with a connection like that, because not so many episodes, Enterprise has its last shot at the Klingons, a recurring foe from the first two seasons but absent from the Xindi third.

Assuming fans made all these connections, "Observer Effect" wastes less time than it seems.  It's also the closest to a Hoshi episode that the season gets.  Yes, that counts as pretty sad.

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Memory Alpha summary.