STAR TREK: TOS #11 - The Yesterday Saga, Book One - Yesterday's Son

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 Sarpeidon, 5000 years ago, Spock knew a beautiful, primitive woman. Now he has gone to meet his son! Our current assignment of charting Sector 70.2 of this unexplored quadrant has been proceeding uneventfully-so uneventfully that I've been resorting to simulated battle drills and abandonship exercises to keep up my crew's efficiency.