Spock, Sulu, and Uhura are taking a shuttlecraft, Copernicus, to deliver a Slaver stasis box to Starbase 25. The Slavers were an ancient race that enslaved the galaxy a billion years ago until their slave races rose up against them and both sides were wiped. The only surviving remnant of the Slavers are the stasis boxes they left behind. Some have contained advanced technology, some have contained booby traps. The Enterprise crew are delivering one discovered by archaeologists on the planet Kzin.
As they pass a stellar formation, Beta Lyrae, their stasis box begins to glow and the glow points to the star. Spock explains that the only known Slaver box detector is another Slaver box. He can't understand how a Slaver box went undiscovered given that Beta Lyrae is a common spot for passing ships to visit. However, he orders them to proceed to the star to investigate. They follow the pointing glow to an icy planetoid with no atmosphere and activate their life support belt before going out to explore. The glow indicates a spot 30 meters below the ice but before they can excavate, several felinoid Kzinti stun them into unconsciousness with phasers.
The Enterprise party wake up aboard the Kzinti ship, confined within a police web. Spock notes that Kzinti aren't allowed weapons or anything other than police vessels under treaty to the Federation, after three prior defeats. The Kzinti commander, Chuft Captain comes in accompanied by his warriors and Kzin Telepath. Uhura notes that Kzinti telepaths are all unhappy neurotics and Spock warns that Kzin Telepath will focus on Sulu since Spock is a pacifist vegetarian and Kzinti women are non-sentient. He suggests Sulu focus on eating raw vegetables if he senses he's being telepathically scanned, and tells Uhura to play dumb and take advantage of the Kzinti ignorance of sentient females.
Chuft Captain only speaks to Sulu and claims to be a privateer with a stolen police vessel. He informs them that he has an empty stasis box that they used to lure in the shuttlecraft with the real Slaver box. He boasts of using the technology inside to defeat the Federation, confirming their suspicion he's working with the Kzinti government. Chuft Captain admits that he pretends independence to provide his government with deniability but it won't matter to the Enterprise party since they won't survive to say differently.
Chuft Captain and his men open the Slaver box and find inside three items: a picture of an alien creature (apparently a Slaver), fresh meat, and a pistol like green object. The Kzinti take the Enterprise party outside and confine them within the police web again. As they prepare to test the Slaver weapon them, Kzin Telepath admits he's reluctant to scan Spock and Uhura. Chuft Captain sympathizes but says it may be necessary. As they prepare to test the Slaver weapon, the technicians confirm the meat is poisonous. Chuft Captain admires the picture of the Slaver, considering it a worthy adversary.
Initially, Chuft Captain is unable to get the device to function. However, he moves a sliding toggle on the handle and the weapon transforms into a gun. He points it at Sulu but only generates a vibration in the metal of the life support belt. Assuming it's some kind of communication device or a stunner for an unknown alien race, Chuft Captain moves the toggle and the Slaver weapon transforms into a telescope. On the next setting it becomes a small laser, but no more advanced than Federation technology. Chuft Captain toggles the device again and it transforms into a rocket pistol. It propels him across the ice, knocking Uhura free of the web and then across Kzin Telepath, breaching the latter's suit. Uhura runs for it and the other Kzinti don't realize anything amiss until Chuft Captain reminds them human females are sentient. They quickly stun her and place her back into the web, while Kzin Telepath is taken into the ship for medical treatment.
As they wait for Chuft Captain to renew testing of the Slaver weapon, Spock notes that none of the settings so far have been superior to known technology. The next and last apparent setting transforms the weapon into an energy absorber. Unaware of its function, Chuft Captain unwittingly deactivates the police web. Realizing their free, the Enterprise trio make a run for it. Spock pauses to kick Chuft Captain to the ground and take the Slaver weapon. He escapes with Sulu but Uhura is stunned and recaptured.
Hiding in the nearby rocks, Sulu figures that Chuft Captain will call for reinforcements. Spock explains that he deliberately attacked Chuft Captain. As a pacifist herbivore, it is now a matter of honor for Chuft Captain to deal with the matter on his own. Sulu concludes that the Slaver weapon is for a spy, not a soldier, as none of the varied functions are something a front line soldier would need. Spock assumes that as such, it would have a self-destruct function. Sulu notes that there are no more settings but Spock conjectures that the initial null setting serves no purpose that couldn't be fulfilled by one of the less dangerous settings. He figures that it may be a key setting to unlock additional features.
The Kzinti ship lifts off and Chuft Captain broadcasts an offer to trade the Slaver weapon for Uhura. Sulu responds and the Chuft Captain offers single combat with Spock as an alternative. The Kzinti says that he has two broken ribs and hasn't sought treatment, but Spock is aware that Kzinti have additional vertical bracing and doesn't estimate his chances of winning as being very high. Sulu turns down the offer and twists and turns at the Slaver weapon in its null setting. It assumes a new weapon-like configuration. Sulu aims it at a distant mountain and fires. The resulting energy beam totally converts matter to energy, causing a large nuclear explosion. The weapon is far in advance of Federation technology and the Kzinti could rule the galaxy with it. However, the shockwave slams into the two men and knocks them unconscious, while the Slaver weapon reverts to the null setting.
The Kzinti recapture and imprison them and examine the Slaver weapon. They finally activate a new configuration: an AI computer. Chuft Captain interrogate the device, but it only says that it was on a secret mission and can't tell them of it without the proper passwords. However, when they ask it for the total conversion beam setting, it readily instructs them how to activate it. Spock notes that it's not the same configuration they used earlier. Chuft Captain and his soldiers go outside to test it on a nearby mountain. Meanwhile, Spock analyzes the situation and concludes that a spy's personal computer, if not given the proper passwords, would logically give them the self-destruct setting. Outside, Chuft Captain pulls the trigger and the device explodes. Part of the ship's hull is cut away, knocking out the remaining soldier and cutting through the police web. The Enterprise trio, freed, return to their shuttlecraft and depart. Spock notes that if the Slaver weapon hadn't been destroyed, the Klingons and others would have fought to acquire it, so it's better off destroyed.