With Captain Kirk in command, the USS Enterprise was hunting for a mysterious freight vessel. The unidentified ship made a break for it by running through an asteroid belt, prompting Kirk to order the Enterprise to shield the craft until its passengers could be brought on board. Unfortunately, the maneuver resulted in the total destruction of all but one of the lithium crystal circuits powering the warp engines.

The Enterprise quickly beamed the captain and three passengers of a cargo ship onto the ship just before their vessel was destroyed by an asteroid. Leo Walsh identified himself as the captain when they arrived in the transporter room. His three female companions were mesmerizingly attractive and captivated most of the Enterprise crew, except for Mr. Spock, who was unaffected by their beauty due to his Vulcan heritage. Ruth Bonaventure, Eve McHuron, and Magda Kovacs were the ladies' names and were meant to be brides for settlers on Ophiuchus III. All three had joined Walsh's crew to break from life with bleak matrimonial prospects.

Kirk summoned a gathering, and the machine disproved Walsh's account, announcing that his vessel license had been annulled and compelling him to divulge his original name, Harcourt Fenton Mudd, a felon with a long rap sheet. The assembly concluded as the last lithium crystal stopped working.

The crew of the Enterprise had to resort to their reserves for energy as they made their way to the storm-ridden Rigel XII to get lithium from the miners. Unfortunately, Ben Childress, the leader of the miners, demanded the freedom of Mudd and the women in exchange for the crystals. But since the ship was on the brink of running out of power, they wouldn't be able to stay in orbit for more than a few days, and therefore, Kirk had to agree to let them beam down to the planet. Nonetheless, Childress went back on his word when they arrived.

During an unexpected get-together with the miners, Eve was infuriated when they started quarreling over the two ladies. She ran into an electric dust storm, with Childress and Kirk chasing her. Kirk beamed back to the Enterprise to seek out their location from space. Childress eventually located Eve, brought her to his room, and fell asleep. When he awoke, he was surprised to find a plainer version of Eve.

Kirk and Mudd transport back down to the planet to face Childress. Captain Kirk reveals that Mudd has been using the Venus Drug, which gives a fake, short-lived beauty, and that the other two miners have already married Ruth and Magda. As Childress confronts Mudd about the deception, Eve takes a dose of what looks like the Venus Drug but is actually a fake. This restores her self-confidence to the level that Childress finds her as attractive as before. Content, he hands Kirk the replacement crystals. Kirk offers to take Eve away, but she chooses to stay with Childress. Eve tells Kirk, "You have someone up there called the Enterprise." The vessel continues its mission with Mudd under arrest.