The U.S.S. Enterprise receives orders to arrest Federation outlaw Harry Mudd, who is accused of selling fake love crystals. Intercepting Harry on the mining colony of Motherlode, they bring him aboard the Enterprise where his love potion gets loose among the crew.

William Shatner (Captain James T. Kirk)
Leonard Nimoy (Commander Spock)
DeForest Kelley (Dr. Leonard H. McCoy)

Roger C. Carmel (Harry Mudd)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lieutenant Uhura)
Majel Barrett (Nurse Christine Chapel/Lt. M'Ress)
James Doohan (Lt. Cmdr. Montgomery Scott/Lt. Arex)

• Roger C. Carmel returns to voice Harry Mudd. He was last seen in "I, Mudd".
  
• Mudd takes advantage of Nurse Chapel's crush on Spock, giving her the love crystal while secretly taking her security card. This is the only episode a security card is shown.
  
• Lt. M'Ress no longer purrs and growls after each sentence. Majel Barrett also speaks with a lower timbre in her voice to differentiate her a bit more from Nurse Chapel.