The plot of “Time’s Arrow” is, as are all time-travel stories, a little complicated. A replica of Data’s head was found in a cave back on Earth, leading the crew of the USS Enterprise-D to investigate. Through a complex series of events, they find a species of pan-dimensional aliens resting in the cave, as well as a time portal that leads back centuries. The aliens have been plundering Earth’s past, sucking energy from the locals and using the energy as food. They’re essentially soul vampires. Data (Brent Spiner) is thrown back in time where he attempts to blend in, and where he meets a younger Guinan and Mark Twain in his heyday. The rest of the Enterprise crew follows Data later on.
It will be Twain who, thanks to Data’s odd appearance and some overheard sci-fi tech talk, begins to suspect that there may be a time traveler in their midst. Given that he wrote “A Connecticut Yankee in King Arthur’s Court” a few years prior to the events of “Time’s Arrow,” Twain is open to the fantastical possibility.
Also, not to disappoint viewers, the writers of “Time’s Arrow” also wanted to feature a scene wherein Twain is brought back to the future where he would be allowed to wander the halls of the Enterprise. In a notable scene with Counselor Troi (Marina Sirtis), Twain finds the post-capitalist utopia of “Star Trek” baffling. He initially assumes that the Enterprise is built to destroy cities and engage in military conquest; he’s heard the “we’re only here to explore” line from too many colonialists in the past.
Troi explains there is no more poverty, and that power is no longer an end unto itself. Twain is impressed and says he’d give up cigars for a future like that.