Killing Blow is the second of three novels in the Errand of Vengeance novel series.
The USS Enterprise is patrolling the Klingon border when sensors detect a massive power source on a planet supposedly populated only by a race of primitive humanoids. Suspecting some sort of Klingon plot, Captain Kirk decides to investigate the matter personally, beaming down to the planet with Dr. McCoy and a team of security officers.
But Kirk is in more danger than he knows. Among the landing party is Lt. Jon Anderson, a Klingon infiltrator on an undercover mission aboard the Enterprise. Anderson does not know if the Empire is at work on the unnamed planet, but if it is, then his duty demands that Kirk be stopped - by all means necessary.