is proceeding through an unexplored sector of space where ships have disappeared every 27.346 years from one of the major empires. Right on cue, Uhura receives an odd subspace signal and pumps it through the ship. Spock determines that they're being probed by the Taurean star system, 20 light years away. All of the male crewmembers believe the signal is a summons although Uhura disagrees. Later, Uhura calls Nurse Chapel to the bridge to observe the male crew members, who hallucinate attractive women. Uhura and Chapel don't see anything and Uhura calls McCoy to the bridge. He's in Sickbay, equally enraptured.
arrives at the planet and determines that there was once a vast civilization. Kirk takes a landing party of himself, Spock, McCoy, and security officer Carver down to the planet. They arrive at a local structure and feel compelled to enter it. Spock warns it may be dangerous but Kirk casually walks in with the others. A group of beautiful women, led by Theela, knows them by name and welcomes them in. Spock determines there are several internal differences despite their humanoid form. Theela explains that the Opto-aud reveals the crew to them. She activates it with tonal control and explains it will reveal whatever is asked. Theela say she'll explain the purpose of the signal later, but asks them to join them at a feast now. The men become increasingly distracted and Theela explains that the males of their civilization occupy another compound. The men suddenly collapse and Theela has them taken to private quarters to rest.
Later, McCoy and the other wake up and discover that they have aged considerably, and there are headbands attached to them that can't be removed.
Aboard the ship, Chapel correlates the results of the scans run by the female crewmembers. The results indicate that the probe targets males and can bring on weakness and possibly death.
Kirk and the others try to convince Theela to release them, but she refuses, has her women overpower them, and return them to their chambers.
Uhura and Chapel go to see Scotty, who is busy singing on the bridge. Uhura is forced to take command of the ship.
The landing party discovers the Taurean women have taken their communicators and phasers, but left them McCoy's medical kit. McCoy injects the landing party with a strong stimulant and then they go to the chamber containing the Opto-aud. Spock opens the door using a medical instrument, but the Taureans spot them and give chase. The landing party flees out into the garden and takes refuge in a large urn. The men notices that the headbands glow when the women are present, and Spock concludes the headbands act as a conduit, transferring their life force to the Taureans. Within four days they'll be dead, and Theela will lure the other male crewmembers down to the planet. Since Spock has a longer life span, he proposes that he go to the temple and try to find their communicators. He makes his way to the temple, activates the Opto-aud, and has it show him where the equipment is. It reveals the equipment is in the main meeting chamber. Spock gets there and calls for an all-female rescue party. The Taureans arrive and Spock collapses as his headband glows brighter.
Uhura and Chapel lead an all-female landing party to the planet and demand answers. Theela tells them to leave but Uhura refuses and the security guards easily subdue the Taureans. As they search the buildings, Spock manages to send a telepathic signal to Christine from a hidden chamber. They find him and are horrified to discover that he has aged even further. Outside, a storm comes up. Kirk, McCoy, and Carver are trapped in the urn as it begins to fill. On the ship, Chapel removes Spock's headband and he tells them to have the female ship's engineer divert all ship's energy into the deflectors to block the probe. Uhura demands that Theela tell her where Kirk and the others are. Theela uses the Opto-aud to explain that the planet drained the energies of the original colonists, and the women evolved to survive by feeding on their male counterpart's life force. Every 27 years they need to revitalize their life force, but they don't age or die. Uhura has Theela use the Opto-aud to reveal where the landing party is. They realize the men are trapped in the urn and Uhura blasts them free just in time.
The men are taken to Sickbay but there's nothing they can do. Spock suggests that they use the transporter, which holds the molecular pattern of their original bodies. He has them reprogram the transporter to restore them to their original patterns before they beamed down to the planet. Scotty beams down to the planet, reprograms the transporter, and beams them up. They start to lose the pattern but Scotty stabilizes it and the landing party is restored to normal. On the planet, Theela destroys the devices they use to drain the life force and Uhura agrees to have them transported to another world where they can obtain normal lives away from the planet's influence.