Lt. Jon Anderson is the newest security officer aboard the U.S.S Enterprise. Starfleet Command fears that the Klingons are gearing up for a major offensive. The Enterprise would be on the front lines of an all-out interstellar war, but Lieutenant Anderson is actually a Klingon infiltrator assigned to sabotage Starfleet from within. His first mission: kill Capt. Kirk.
Sensors detect a massive power source on a planet supposedly populated only by a race of primitive humanoids. 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, as among the landing party is Lt. Jon Anderson, a Klingon infiltrator on an undercover mission.
In anticipation of a monumental conflict with the Klingon Empire, the Enterprise is assigned to guard a vital starbase located perilously close to Klingon space. But even as Kirk's mission brings him into a tense confrontation with an invading Klingon battle cruiser, an equally deadly menace lurks within the ranks of his own crew: Klingon infiltrator agents.