The Enterprise travels to the center of the galaxy only to discover themselves sucked into a dimension where the laws of time and space no longer apply, and "magic" functions for those capable of grasping its fundamentals. Stopping at Megas-Tu, a planet where magic and witchcraft are the norm, the crew finds their guide is Lucien, who is Lucifer of Earth mythology.

Harry Fenton Mudd is once again apprehended and incarcerated by the U.S.S. Enterprise crew, after trying to sell love crystals. He claims the love potion can cause any male and female to fall in love. He inadvertently releases it into the Enterprise's ventilation system and they discover that his claims are true. Mudd abducts Nurse Chapel and briefly escapes to a rocky planet in a stolen Enterprise Heavy shuttlecraft, but he is recaptured and sentenced to an indefinite period in rehabilitation therapy for violations of the Federation pharmaceutical code, including fraud, illegal drug manufacturing, swindling, and transport of a dangerous life-form (a Rigellian hypnoid).