Enemy Unseen

Summary
From the back cover: Transporting a diplomatic party is nothing new for Captain James T. Kirk and the crew of the Enterprise - but this particular mission promises trouble from the start.

For one thing, the wife of the Federation ambassador on this trip is an old flame of Kirk's - and she's determined to see that they resume their romance where they left off. Of course, when another ambassador presents Kirk with three of his wives, finding time for the first romance, let alone any of his other duties, is going to prove nearly impossible.

When a diplomatic attache is murdered, and the prime suspect is one of his crew members, Kirk begins to wish that Starfleet Command would consider using some other starship to ferry diplomatic personnel...