Harcourt Fenton Mudd
Roger C. Carmel

Eve McHuron
Karen Steele

Magda Kovacs
Susan Denberg

Ruth Bonaventura
Maggie Thrett

Ben Childress
Gene Dynarski

Lt. John Farrell
Jim Goodwin

Benton
Seamon Glass

Mr. Connors
Eddie Paskey

In pursuit of an unidentified vessel, the sensors of the Enterprise show that the object of its chase is in danger of breaking up. Kirk pushes his vessel to the limit, beaming the occupants aboard in the nick of time, but causing damage to his own ship's dilithium crystals in the process. The rescued 'crew' turn out to be three gorgeous females and galactic-class entrepreneur, smuggler and con-man Harcourt Fenton ( Harry ) Mudd who claims to be transporting the ladies as wives-to be for settlers at a remote outpost.

Harry's record is uncovered in the ship's library computer and he is promptly confined to quarters. In order to repair the starship's engines, Kirk heads for Rigel XII, a lithium-mining operation occupied by three lone miners, a fact that gets Harry Mudd to thinking how he can turn it to his advantage. He gets one of the women to steal a communicator and makes a deal with the miners so that, arriving at the planet, Kirk is presented with an ultimatum - turn over Mudd and the women or no crystals.

Unknown to the miners, the women's appearance and attraction is due to Harry supplying them with the 'Venus' drug which transforms them, from their normal plain, run-down look, into goddesses. The women are allowed to accompany the miners down to the planet but one of them, Eve, becomes disenchanted with her partner and runs off. The miner chases after her through a storm and rescues her, but soon finds that she is not all that she appears to be when the drug's effects wear off.

Kirk and Mudd beam down to the surface where she is supplied with a pill and turns back into her beautiful state right before their eyes. Kirk reveals to her that what she has taken is no more than coloured gelatine and that her apparent change was due to her belief in herself. Happier, she decides to go back to the miner, which prompts him to allow Captain Kirk to have the crystals he needs for engine repairs. The Enterprise leaves the women on Rigel XII with the miners but Harry Mudd is taken along to face prosecution for his outstanding misdemeanours. Kirk promises to act as a character witness for Mudd at his trial prompting the final words from Harry . . ."They'll throw away the key . . . "