With Spock stricken down by a fatal disease, the Enterprise must track down the drug that can cure him before it is too late.

The Enterprise crew is joined by a unpredictable alien called Ari bn Bem, an independent observer from the planet Pandro.

The computer of the Enterprise gains partial sentience and starts playing practical jokes on the crew.

Dr. McCoy is arrested and held responsible for causing a plague that ravaged the planet Dramia II, where he performed a mass-innoculation 19 years earlier.

The Enterprise runs into a being that once visited Earth and influenced the Mayan culture.