Spock, Sulu, and Uhura are aboard the shuttlecraft Copernicus
en route to Starbase 25. They are delivering a Slaver stasis box, discovered by archaeologists on the planet Kzin. The Slavers were an ancient race who ruled the galaxies a billion years ago. The only source of knowledge about them is their stasis boxes that they left behind. Their stasis box starts to glow and Spock explains that it indicates the presence of another Slaver stasis box. The glow indicates Beta Lyrae, a rare stellar formation. Spock is skeptical that no one else has ever found a stasis box given it's an oft-visited phenomenon. Nonetheless, Spock orders Sulu to set course for Beta Lyrae to investigate. As they proceed, Spock notes that some slaver boxes have contained advanced technology and others contain undetonated explosives.
They land on a small planetoid. Activating their life support belts, the three crewmen proceed to explore and determine the other box is 30 meters below the ice. Sulu prepares to melt the ice with his phaser but spacesuited Kzinti warriors attack and stun all three of them with phasers. They recover consciousness aboard a stolen police vessel, held within a police web. Spock notes that the Kzinti aren't allowed weapons under treaty provisions. Chuft Captain, their leader enters along with a Kzinti telepath, an unhappy neurotic. Spock warns that the Kzinti will focus on Sulu since they consider vegetarians and women inferior beings. He suggests Sulu think of eating raw vegetables if the telepath tries to scan them. Spock also tells Uhura to act dumb and play into the Kzinti's prejudices. Chuft Captain addresses Sulu as an omnivore and identifies himself as a privateer. He boasts of using an empty stasis box to lure them there and wants whatever advanced technology their stasis box may hold. He believes they can gain superior weaponry and defeat the Federation, confirming he's secretly working for the Kzinti government. If they're captured, the Kzinti government will repudiate them.
Chuft Captain has his technician open the stasis box. Inside they find several items: a picture of an alien creature that may be a Slaver, fresh meat, and a pistol-like object. The Kzinti move the Enterprise
crew to the surface to test the apparent weapon. Kzin Telepath admits it's reluctant to scan Spock and Uhura's minds but Chuft Captain notes it may be necessary. On the surface, the Enterprise
crew are placed back within the police web and a technician reports that the meat found was poison. The telepath confirms that Sulu believes the picture to be a Slaver. Chuft Captain pulls the trigger on the device but nothing happens. He moves the toggle and the weapon transforms but seemingly does nothing except generate a slight vibration. Chuft Captain suspects it may be a communication device or a sonic stunner. He transforms the weapon again and discovers it functions as a telescope in that configuration. On the next setting it becomes a small laser, no more advanced then anything the Federation has. The next configuration results in a small handheld rocket. The Chuft Captain flies out of control, knocking Uhura out of the police web. She makes a run for it and the Kzinti initially ignore her. Chuft Captain passes by Kzin Telepath, rupturing his suit. He orders his men to subdue Uhura and they finally stun her.
Uhura is placed back in the web and Kzin Telepath is taken back to the ship for medical treatment. Uhura admits she's slowing down but didn't miss much. Spock notes none of the settings so far have been superior to Federation technology. The next setting creates an odd spherical device with no gunsight. Chuft Captain pulls the trigger but fails to notice that the lights and police web have been drained of energy. Spock, Sulu, and Uhura make a run for the shuttle. Sulu and Uhura run while Spock kicks Chuft Captain, disarms him, and escapes with the Slaver weapon. The Kzinti stun and recapture Uhura while Chuft Captain retires to the ship. Sulu notes they're trapped and the Kzin can call for reinforcements, but Spock notes that he, a pacifist vegetarian, attacked Chuft Captain. The Kzin captain will consider it a matter of honor to settle the matter himself. Sulu, a weapons expert, concludes that the item is a spy's weapon. None of the functions are necessary for a soldier. Spock believes it has a self-destruct setting but Sulu notes they've seen all the settings. Spock speculates the null setting is a key to another setting.
Chuft Captain lifts off in his police vessel and announces that he will trade Uhura for the Slaver weapon. Sulu responds and Chuft Captain offers Spock single combat, saying he has two broken ribs that he has not sought treatment. Spock notes that Kzin ribs have vertical bracing and it's unlikely he can defeat him. Sulu refuses and experiments with the Slaver weapon's null setting. He finds a new configuration with a gunsight. Sulu aims it at distant mountain and fires, and the resulting total matter-conversion release destroys the mountain. They realize the Federation has no such technology and the Kzin can't be allowed to use it. The shockwave from the explosion knocks out Sulu and Spock and Chuft Captain recaptures them. However, the Slaver weapon has reverted to its null setting. Chuft Captain tries to find the total conversion setting and triggers a new conversion: a spherical device with a speaker grille. The device activates and addresses the Kzin: it's a reasoning computer. Chuft Captain demands answers from it but it has no idea of how much time has passed. It remembers being on a secret mission it can't speak of without being given code words. Chuft Captain demands the total conversion setting and the computer obliges. However, the resulting configuration is an entirely new one.
The Kzin take the device outside and prepare to fire it at a distant target. Spock concludes that the Slaver war computer assumed that the unknown individuals interrogating it are enemy soldiers. When it was asked for its most powerful weapon setting, they figure it will provide the self-destruct. Outside, Chuft Captain pulls the trigger and the resulting explosion destroys him, his soldiers, and a large chunk of the police vessel. The Enterprise
crewmen activate their force belts and Spock allows that the total conversion beam was the only technology the Federation doesn't already possess. He notes that the weapon is better off destroyed, as other races would have sought out the device for the total conversion setting. As they head back for the Enterprise
, Uhura notes that the Kzinti are never going to overcome their superstitions about haunted weapons.