Dr. McCoy and Mr. Spock find themselves trapped in a planet's past Ice Age, while Capt. Kirk is in the same planet's colonial period. However, it's the former pair that has the most trying time. Besides the freezing temperatures and sanctuary to be found only in caves, there is a third inhabitant, the beautiful and so sexy Zarabeth (Mariette Hartley). As Spock spends more time in this era, he slowly begins to revert to the behavioral patterns of his ancestors, feeling a natural attraction to Zarabeth and throwing "caution to the wind" about ever leaving this place. Only with Dr. McCoy's constant "reminders" does Spock hold on to some grasp of reality.<br /><br />This stand as one of the few times when the character gets to show some "emotion" and Nimoy (Spock) plays it to the hilt, coming close to knocking the bejesus out of Deforest Kelly (McCoy). Surprising to previous installment, Captain Kirk (William Shatner) wasn't allowed to get the girl, another plus for this one.<br /><br />Perennial "old man" Ian Wolfe assays the role of "Mr. Atoz," the librarian responsible for sending the trio into the past.