The Enterprise has been sent to explore an uncharted area of space. The Federation has conferred with the Klingon and Romulan Empires and determined that at least one ship has disappeared in the area 27.346 years. As they arrive at the designated time, Uhura picks up a subspace signal and broadcasts it throughout the ship. Spock confirms that it is some kind of probe and originates from 20 light years away in the Taurean system. The male crewmembers all become entranced and start hallucinating beautiful women, and feel that the signal is some kind of summons. Uhura doesn't see or sense anything and calls Nurse Chapel to the bridge. Chapel is equally unaffected and they try to summon McCoy to the bridge, only to discover he is bedazzled as well.
The Enterprise enters orbit above Taurus II. Spock, just as affected as the other male crew members, manages to scan the planet and determines that it was once home to a vast civilization. Kirk beams down to the planet with Spock, McCoy, and security officer Carver. They find several attractive local structures and immediately enter the nearest one despite Spock's half-hearted warnings.
A group of beautiful women greet them and ooh and awe over the males. The women's leader, Theela, knows all of them by name. As she greets them, Spock scans the women with his tricorder and discovers several internal discrepancies. Meanwhile, Theela tells them that she learned of them using a piece of ancient technology, the Opto-aud. She activates it by humming a particular tune, and demonstrates that it will reveal anything asked of it. When the landing party asks about the probe signal, Theela puts off an explanation to later and asks them to participate in a feast in their honor. The landing party wonders why there are no men and Theela says that the males of their species occupy another compound. When one of the female entertainers throws a fruit to Spock, he tries to catch it and collapses, and the other three men soon pass out as well. Theela tells her people to take the Enterprise men to private quarters.
Later, Kirk wakes up in a darkened room and the others come in. They are each now wearing a metal headband and have aged considerably. When Kirk looks in the mirror, he sees that he is equally affected.
Aboard the Enterprise, Chapel has the female scientists scan and analyze the probe signal. The results indicate that it affects male and drains energy from them, resulting in weakness and possibly death.
The now severely aged Kirk asks Theela to return their equipment and release them. She refuses and orders the women to overpower the men, which they easily do.
Uhura and Chapel report their findings to Scotty, but he is still entranced by the probe signal. Uhura takes command of the ship and Scotty readily obliges her.
Back in their quarters, the Enterprise men discover that the Taurean women left McCoy his medikit, apparently because they didn't think it was of any use to them. McCoy gives everyone stimulants and they try to get to the chamber containing the Opto-aud so they can find their equipment. They manage to get the door open but the Taurean women spot them. The men escape out into the garden but in their weakened condition can't get far. They take shelter in a large urn-like sculpture and grow even weaker as the headbands glow brightly. When the women leave, the glow dims again. Spock believes that the headbands are conduits, channeling their life force to the women. Spock's calculations indicate that they will be dead within four days, and once that happens the Taurean women will lure down more of the Enterprise crew. Spock suggests that he return to the Opto-aud and find their equipment, since he has a relatively longer lifespan. The others have no choice but to agree.
Spock enters the main structure and makes his way to the Opto-aud. It shows him their equipment, hidden in a chair in the great hall. He gets there but his headband starts to glow. Spock uses a communicator to call Uhura for help, specifying an all-female landing party to come to the rescue. As Theela and the others arrive, Spock collapses.
Uhura takes Chapel and a security team down to the party and confronts Theela. When the Taurean leader tell them to go, Uhura refuses and orders her people to stun the Taureans. They search the building without success, unaware that Spock is hidden in a secret chamber. Hearing them, he manages to send a telepathic message to Chapel that he's in the vicinity. Hearing him, she finds the secret chamber, but is horrified to see that he has aged even further and is on the point of death.
While Uhura and the security team continue their search for the other men, Chapel beams up to Sickbay with Spock and removes the headband. Spock stabilizes but is still in critical condition. He tells Chapel to have the female engineer in charge divert all but essential power to the ship's shields to block the probe signal.
On the planet, a storm comes up and the urn starts to fill with rainwater. Unable to find them, Uhura orders Theela to provide her with answers. Using the Opto-aud, Theela demonstrates how when they colonized the planet, radiation altered their bodies and drained the energy from males. The women evolved a method to tap into the energy, but in the process were rendered both immortal and sterile. The Taurean women need to restore their life energies once every 27.346 years, and channel the signal to lure in a starship containing men. Uhura tells Theela to use the Opto-aud to find the men and it shows them drowning in the urn. Running outside, Uhura uses her phaser to burst open the urn just in time.
The men are beyond any medical treatment but Spock suggests they use the transporter. He speculates that since it stores their molecular patterns, they can use it to recreate themselves at a point prior to their last transport. With no other choice, Scotty beams the landing party down and then attempts to beam them up while reintegrating their original transport patterns. The patterns shift briefly and are almost lost, but Scotty manages to stabilize them and the men are restored to their original bodies.
Uhura makes an agreement with Theela: the Taureans destroy the life-draining devices and the Enterprise will be used to transport the women to a different planet where they can be restored to normal, and have the chance to have normal relations with men.