And so the serviceable but uneven fifth season ends — with a serviceable but underwhelming "cliffhanger" that continues in the obligatory tradition of season-enders but definitely doesn't live up to the label in terms of its impact or suspense. After a number of memorably good and great episodes the second half of this season, "Time's Arrow" is a letdown, showing indications the writers took an average concept and tried to twist it into a two-part "event."
An archeological discovery finds Data's detached head, — yes, his head — buried in a cave beneath San Francisco. It has apparently been there for 500 years. Data concludes that in the future he will travel back in time where he will be destroyed in the 19th century. There's a certain intrigue and poignancy in watching Data matter-of-factly ponder the inevitable future of his own death, which up to this point was never guaranteed. Also found in the cave: traces of This Week's Technobabble, "triolic rays," which the Enterprise traces back to the planet Devidia II. There the investigation into the triolic rays reveals possible human life, but the readings are all phase-shifted because of a mysterious alien presence; Data uses a device to shift himself out of phase to further investigate, but then he gets pulled through a temporal vortex where he winds up in the late 19th century.
The scenes in the 19th century are where this story really kind of stalls out. Data has an extended conversation with a sick old 49er, then with a hotel bellhop (Michael Aron), and then plays a poker game (one of the players is Marc Alaimo, aka Dukat) in order to win some money to secure a hotel room and buy equipment for running his technobabble investigation via 19th century science. About this time he sees Guinan (!) in a newspaper story for a literary reception. (Back on the Enterprise, Guinan insists to Picard, with vague overtones of cosmic urgency, that he must go on the away mission to the planet surface.)
Mark Twain (Jerry Hardin) is among the noted guests at the literary reception, where he gives an extended speech about the significance of humanity in the universe (with Hardin chewing the scenery at length) that I'm afraid the story thinks is far more amusing and significant than it really is. (I'm not a literary historian and have no idea if Twain was this much of a showboat and an eccentric, but here his personality is out of scale compared to everything else.) The 19th century scenes are all sluggishly paced, and San Francisco just feels too much like a Hollywood backlot.
"Time's Arrow, Part I" is all setup and absolutely no payoff (as opposed to "Mr. Worf, fire," which is its own payoff). The final scene of the season has the crew following the mysterious aliens (who are apparently soul-eating time travelers!) through their temporal vortex and into the 19th century, but the execution is so lacking in juice and urgency that the music has to futilely sell us on how "significant" this ending is. I suppose this is okay as a whimsical hour of conventional time-travel sci-fi, but it's severely lacking as a season-ending hook.