As a landing party prepares to beam down to a previously unexplored Class-M planet, a beautiful woman, Losira, appears. She touches an ensign and kills him. Already dematerializing in the transporter beam, Kirk and the rest of his party are helpless to stop her. Losira then disappears.
The surge of power that Losira's appearance caused hurtles the U.S.S. Enterprise 990.7 light years away from where they were. Spock calculates that it will take 11.33 hours at warp 8.4 to return to the planet.
On the surface of the planet, Kirk, McCoy, Sulu and geologist D'Amato discover the vegetation is poisonous to humans and the rocks are made of an alloy that did not develop naturally. Losira appears again and kills D'Amato, again, by merely touching him. She vanishes, then reappears, this time for Sulu, but he avoids her and they realize she can only harm the person whose name she calls.
Kirk, Sulu and McCoy band together to keep her from killing them. Losira reappears on the Enterprise, in engineering, and kills another crewman. When Scotty insists that something "feels" wrong with the ship, they discover that Losira had sabotaged the matter/antimatter integrator. Scotty repairs the device before it has a chance to explode.
On the planet's surface, the landing party finds a chamber in the rocks that houses a computer. They realize that this is where Losira appears from. Losira appears to them again, this time in threes, so that she can touch each officer at once.
Spock and a security team arrive and destroy the computer that was projecting Losira's image. A visual recording triggered by the computer shows that the planet was once an outpost of the Kalandan race. It was ravaged by a deadly organism which supply ships unknowingly carried back to their home world. With the computer destroyed, the last of the Kalandans' civilization is also dead.