The new security officer, Lt. Jon Anderson, is actually a Klingon infiltrator by the name of Kell. He is surprised to find that there is another Klingon onboard the Enterprise, masquerading as Ethan Matthews, but Matthews gets killed during the mission to Exo III, and his body is lost, so his true identity is not revealed. Then a plea for help comes in from a group of anti-Federation human settlers, whose colony, close to the Klingon border, is under attack. It is believed that Klingons are preparing for an attack on the Federation, and Admiral Justman has brought Ensign West onto his staff to use xenoanthropology techniques to try to find a way (or ways) to respond to the Klingons. When the Enterprise arrives at the colony world, System 1324, they find that Orions, not Klingons, are the people carrying out the attack. While the Enterprise is drawn away, chasing an Orion ship, a team of security officers, including Kell/Anderson, is sent down to rescue the colonists. The security officers find that they have walked into a trap, but with some ingenuity and the loss of about a third of their number, they rescue the colonists and hold off the Orions. The Enterprise has been partially disabled when the Orion ship they have been chasing self-destructs, but Kirk takes a shuttlecraft back to the planet just in time to prevent the Orions overrunning the remaining security officers and colonists. From his experiences fighting alongside the security officers, Kell has had to readjust his thinking about humans, and is beginning to respect them, even to respect Kirk, although he knows it his job to kill him. Meanwhile, Kell’s brother, Karel (a weapons officer aboard a Klingon warship), has been trying to locate Kell, and has been informed that he is dead, killed by humans.