Yesterday's Son — A.C. Crispin

The Romulans attack the planet Gateway, where Federation scientists are studying the Guardian of Forever--the mysterious portal to the past. The Enterprise must protect the Guardian-- or destroy it. But Spock has already used the portal to journey to the past. On the planet Sarpedion, 5,000 years ago, Spock knew a beautiful, primitive woman. Now he has gone to meet his son!