With Commodore Robert April (the first commander of the Enterprise) on board, Kirk pursues a ship plunging into a supernova, and enters a universe where time runs backwards.

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

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

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

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