The Enterprise has come to the Arcadian star system to locate an "old friend," Harcourt "Harry" Fenton Mudd. Their mission is to corral Mudd for fraud, illegal drug manufacturing, and swindling. On the planet Motherlode, Mudd attempts to sell heavy metal miners a magical love potion, guaranteed to make members of the opposite sex fall in love with one touch. Captain Kirk and Mr. Spock take Mudd into custody before the swindled miners can retaliate.
On Enterprise, Mudd is put into the brig, but he takes advantage of Nurse Chapel's obvious affection for Mr. Spock by offering her the love potion for free. Chapel doesn't believe the potion works, but it just takes longer to break through Spock's Vulcan metabolism. Mudd escapes, taking Chapel hostage and departing the ship on a shuttle, but not before accidentally releasing multiple crystals of his love potion into the ship's air system conduits.
As Kirk and Spock beam down to an arid planet to rescue Chapel from Mudd, they're unaware that the crew of the Enterprise has been infected by the love potion. On the bridge, as Lieutenant Arex strums love songs on his green lute, Mr. Scott and Lieutenant M'Ress make a love connection, and even the male crew members express their deep friendship for each other. On the planet below, the quartet is imperiled by two huge rock creatures, and even as they fight the effects of the love potion, Kirk and the others find a way to stay alive.
On the Enterprise, the potion has dissipated, leading to a period where the crew members intensely dislike each other. And Mudd, out of ploys to free himself, begins a long confession.