The Enterprise receives a signal from a nearby world that entrances all the men and draws them toward the planet, while the women on the ship (represented by Uhura and Chapel) are left unaffected. The transmission's source might more aptly be described as "sirens," with the point driven home by the goofy grins on the faces of the men that have heard them.
The landing party (Kirk, Spock, McCoy, and an irrelevant redshirt who I don't think dies but might as well have) beams down to the surface of the planet and finds a temple filled with women, led by Theela, who says she will explain everything later. In the meantime, they enjoy their time at the palace, which is every man's supposed dream. "The women radiate delight," says Kirk in his log. While distracted by their bliss, Theela has the men "obstructed" with some sleep-inducing nectar; they wake up wearing headband devices that have somehow aged them all, turning half of the plot into "The Deadly Years."
"The Lorelei Signal" is a nonsensical cheese-fest, with an impenetrable plot where things happen and there's no driving force as to the how or why until it's all explained at the end of the hostage crisis, at which point our interest has long since vanished. The magical and brilliantly named "Opto-Aud" (a viewscreen that answers all your questions about the plot, should only you ask) is a classic TOS plot device — divine, convenient, inexplicable, and beyond the scope of the story's desire to deal with. Meanwhile, we get bizarre, time-wasting scenes like the one where Scotty sings while we watch the Enterprise slowly orbit the planet.
On the plus side, we do get Uhura taking command of the Enterprise (something that never happened on TOS or in the movies) and leading the away mission, along with Chapel and an all-women landing party, to save Kirk and the others. This plays almost as a commentary on the other half of the story, where the sirens have ensnared the men with their G-rated implied sexiness. But it can't overcome the pure hokiness of the story's execution and the simplistic tidiness of its resolution.
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