The Enterprise has, once again, gone far too long without shore leave, without repair. They are sent on a diplomatic mission to the planet Perry, a long-lost colony that has expressed a desire to join the Federation.

Arriving at the planet, they find that there is no violence of any kind. In fact, when Sulu, angry at a crewmate, attempts a violent act, he passes out on the spot. He's taken back the ship for treatment, thus infecting everyone there with the so-called peacekeeper virus which maintains order on the planet.

The founder of the colony, Wayne Perry, found that he had a deadly disease (of which megalomania was a symptom, incidentally) while in transit to the colony. He created a computer with an android body, clothed in holograms, to lead the colony in his absence–guided by an interface to the dying Perry's body. The computer, finding itself in need, took on the role of permanent leader of the planet.

With nothing but time on its hands, the computer, doubtless influenced by the broken mind of Perry, concocted the peacekeeper virus in an attempt to perfect his society. Now that the Enterprise had arrived, it plans to extend this 'benevolence' to the whole galaxy.

However, the virus is not perfect. There are Immunes on the planet, who live outside of society, taking whatever they desire with no one able to offer any resistance. They capture Uhura and Dr. Kelly Davis, and take them underground–where the computer is.

In the end Kirk, Spock, and McCoy rescue the ladies, extract a sample from the Immunes with which to develop a cure, and destroy the computer. Along the way, a Klingon B-plot is foiled.

All in all, Perry's Planet is mildly amusing but treads no new ground:

• Once again, we have a computer controlling society, to the detriment of its members, as we saw in "The Return of the Archons" and "The Apple", or perhaps "A Taste of Armageddon", in which the computer's purpose is also to prevent violence.
• There is a peace-inducing fungus in "This Side of Paradise".
• The computerized Perry tries to get Spock hooked up to it, like in "Spock's Brain".
• The computer is malfunctioning due to its association with a human, as in The Ultimate Computer.
• The guild system is reminiscent of World Without End, maybe.

The structure of the thing is familiar, too. There's Klingons attacking to provide some pressure, ultimately leading to a clock counting down to the failure of the dilithium crystals, and thus the destruction of the Enterprise. It feels like an episode of the show, or like any other middling Trek book.