Aboard the Shuttlecraft Copernicus, Commander Spock and Lieutenants Sulu and Uhura are en route to Starbase 25, to which they are to deliver an exceedingly rare technological device: a stasis box constructed by the long-vanished spacefaring civilization known as the Slaver Empire. About one billion years ago, the Slavers conquered and ruled most of the galaxy, until they became casualties of interstellar war themselves. During their heyday, the Slavers often carried valuables — including highly advanced weaponry — inside stasis boxes, in which time stands still; some previously discovered stasis boxes have contained technologies capable of revolutionizing science. Stasis boxes also constitute the only means, other than pure chance, of finding other stasis boxes.
During the voyage to Starbase 25, the stasis box aboard the Copernicus begins reacting to the presence of a second stasis box somewhere nearby, and Spock takes the shuttlecraft to an inhospitable, icy world in the Beta Lyrae system to investigate. After determining that a second stasis box lies some thirty meters below the planet's frozen surface, Spock, Sulu, and Uhura are surprised by a group of Kzinti — sentient, meat-eating, catlike predators — who take them captive. The Kzin reveal that they possess an empty stasis box which they have stolen from a museum — and which they have used as bait to lure another stasis box into their clutches, one they hope will contain a weapon capable of restoring the Kzinti Empire to the power and influence it had enjoyed prior to the Kzinti race's several fateful run-ins with humans over the past two centuries.
Chuft Captain, the leader of the Kzinti raiders and commander of a purloined police vessel now called The Traitor's Claw, finds several innocuous items inside the stasis box, as well as a pistol with several power settings. Aware of how dangerous this item could be in the wrong hands, Spock, Sulu, and Uhura attempt to escape and recover the weapon, which contains an artificial intelligence that insists on being given a prearranged code word before activating. With Sulu's assistance, the Slaver weapon blows itself up rather than be misused by the Kzinti raiders, whom it destroys along with The Traitor's Claw. As the Enterprise officers return to the Shuttlecraft Copernicus to resume their journey to Starbase 25, Sulu expresses regret that the Slaver weapon couldn't have been salvaged. Spock points out, however, that the device may have been too dangerous for anyone to keep.