Harcourt (``Harvey'') Fenton Mudd, attempts to evade the Enterprise with his small class J cargo ship, and leads it into an asteroid field. The Enterprise extends its shields over Harvey's ship, in the process burning out three of its four lithium crystals. Just before Mudd's cargo ship is destroyed by an asteroid, Harvey and his human ``cargo'' are beamed aboard the Enterprise.
On the Enterprise, Mudd pretends his name is Leo Francis Walsh, but is eventually forced to admit his true identity. The crew of the Enterprise, including Sulu and navigator John Farrell, become fascinated with the three beautiful women Mudd has been transporting: Ruthie (from a planet of sea ranchers), Magda (from Halium Experimental Station), and Evie McCurin (from a farm planet). The women are destined to be wives for settlers on Ophiuchus 3. McCoy notices a strange effect on his medical panel when one of the women walks near it and is about to investigate when he is distracted by another matter.
As a result of the destruction of three of its lithium crystals, the Enterprise is forced to divert to Rigel 12 to obtain new crystals. Before it can arrive, Mudd obtains a communicator and make his own bargain with the lithium miners (Chief Ben Childress, Benton, and Herm Gosset) on the planet. At Mudd's prompting, the miners offer to provide Kirk with lithium only in exchange for Mudd's freedom and the three women. Mudd and the women beams down to Rigel 12, where Evie becomes fed up with her plight and runs away, but Kirk finds her without difficulty using ship sensors. In the meantime, he learns the secret to the women's beauty: Mudd has been providing them with the Venus drug. Childress rescues Evie, but wants nothing to do with her when her beauty wears off. Kirk beams down to collect the lithium from Childress, at the same time providing Evie with red gelatin she believes to be the Venus drug. Evie believes herself once again to be beautiful, and unintentionally reveals her natural inner beauty. In the end, Kirk gets his lithium, Evie remains with Childress, and Mudd is taken into custody to be turned over to the authorities, only to make a second appearance in the episode I, Mudd.