Captain Kirk finds out about Pon Farr

[Spock's quarters]
(Spock is staring at the picture of a young girl on his monitor, but switches it off when the doorbell buzzes.) 
SPOCK: Come. 
KIRK: Stay. McCoy has given me his medical evaluation of your condition. He says you're going to die unless something is done. What? Is it something only your planet can do for you? Spock! You've been called the best first officer in the fleet. That's an enormous asset to me. If I have to lose that first officer, I want to know why. 
SPOCK: It is a thing no out-worlder may know except those very few who have been involved. A Vulcan understands, but even we do not speak of it among ourselves. It is a deeply personal thing. Can you see that, Captain, and understand? 
KIRK: No, I do not understand. Explain. Consider that an order. 
SPOCK: Captain, there are some things which transcend even the discipline of the service. 
KIRK: Would it help if I told you that I'll treat this as totally confidential? 
SPOCK: It has to do with biology. 
KIRK: What? 
SPOCK: Biology. 
KIRK: What kind of biology? 
SPOCK: Vulcan biology. 
KIRK: You mean the biology of Vulcans? Biology as in reproduction? Well, there's no need to be embarrassed about it, Mister Spock. It happens to the birds and the bees. 
SPOCK: The birds and the bees are not Vulcans, Captain. If they were, if any creature as proudly logical as us were to have their logic ripped from them as this time does to us. How do Vulcans choose their mates? Haven't you wondered? 
KIRK: I guess the rest of us assume that it's done quite logically. 
SPOCK: No. No. It is not. We shield it with ritual and customs shrouded in antiquity. You humans have no conception. It strips our minds from us. It brings a madness which rips away our veneer of civilisation. It is the pon farr. The time of mating. There are precedents in nature, Captain. The giant eelbirds of Regulus Five, once each eleven years they must return to the caverns where they hatched. On your Earth, the salmon. They must return to that one stream where they were born, to spawn or die in trying. 
KIRK: But you're not a fish, Mister Spock. You're 
SPOCK: No. Nor am I a man. I'm a Vulcan. I'd hoped I would be spared this, but the ancient drives are too strong. Eventually, they catch up with us, and we are driven by forces we cannot control to return home and take a wife. Or die. 
KIRK: I haven't heard a word you've said, and I'll get you to Vulcan somehow.