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).

The U.S.S.Enterprise is sent on a mission to explore a "Bermuda Triangle of Space," only to find itself and a Klingon battlecruiser trapped in a time warp after being caught in an ion storm. The two ships must cooperate before their only exit back to real space closes for good. This Delta Triangle is inhabited by other lost ships, many centuries old.