On Star Date 4513.3, the Enterprise crew is sent to the planet Mudd to investigate a distress signal. Upon arriving, Captain Kirk, Spock, and Dr. McCoy beam down to the planet and meet Harry Mudd, a conman and smuggler. Mudd had been stranded on the planet after his crew deserted him and was desperate to get off.

The Enterprise crew discovers that the planet is inhabited by androids under the control of Mudd's partner Norman. The androids are programmed to ensure Mudd's survival, but they have no loyalty to him.

The female crew members, Yeoman Rand and Uhura, play an essential role in this episode. Rand distracts Norman, while Uhura uses her knowledge of the androids to solve the crew's problem. The actresses who played Rand and Uhura were Grace Lee Whitney and Nichelle Nichols, respectively.

The episode's antagonists are Mudd and Norman, who attempt to manipulate the Enterprise crew. Mudd's plan is to make the androids believe that the crew is their new master, and he attempts to gain control of the ship. However, Norman tries to convince Mudd to forget his plans and stick with him.

Ultimately, Spock can outsmart Mudd and Norman by programming the androids to obey them instead. After Mudd and Norman are arrested, the Enterprise crew leaves the planet.

At the time of this episode, Grace Lee Whitney had already achieved fame in the entertainment industry. She appeared in several television shows, including The Alfred Hitchcock Hour, Burke's Law, and Dragnet. She also featured in several movies, such as The Young Philadelphians and The Great Imposter. Nichelle Nichols had just started out in the entertainment industry, but she went on to appear in various productions, such as Star Trek: The Animated Series and Heroes.

The episode was filmed at Paramount Studios and was the first episode in which the character of Harry Mudd appeared. It was also the first episode of Star Trek that featured a computer-generated special effect.