As our journey through the original "Star Trek" comes to a penultimate close, we're thrilled to be joined for our deep dive of "All Our Yesterdays" by returning guest Adam Nimoy, followed by our exclusive interview with the man himself: William Shatner!

While investigating the disappearance of an entire civilization on a planet whose star is about to explode, Kirk, Spock and McCoy accidentally go back in time and become trapped in that planet's past. Captain Kirk finds himself accused of witchcraft while stuck in a time period resembling old England, while Mr. Spock and Dr. McCoy are transported back to the planet's Ice Age and are given shelter by a lonely woman. With less than a few hours to go before the planet's star goes nova, Kirk must find his way back through the time portal to save Spock and McCoy, but the effects of the time period may have affected Spock to the point where he doesn't want to be saved. With the run of the original series coming to a close, "Star Trek" had one final creative burst of greatness with "All Our Yesterdays," a deeply engaging and entertaining episode that's filled with high stakes, gripping intensity, a touching romance and stellar performances (especially from guest star Mariette Hartley). But the high points are undoubtedly the scenes between Spock & McCoy, for after years of simmering tensions between them, their relationship came to a dramatic boil -- and Leonard Nimoy and DeForest Kelley played these scenes perfectly.

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