Spock contracts a fatal illness, and the cure can only be found with dangerous Orion pirates.

The Enterprise crew is taken captive by a race of primitives on a newly discovered planet.

A strange energy field causes the Enterprise computer to play practical jokes on the crew, but the humor soon turns to danger.

Doctor McCoy is arrested for allegedly causing a deadly plague which once ravaged the planet Dramia.

A mysterious being threatens to destroy the Enterprise if the crew is unable to solve an ancient puzzle.

An unusual spaceship pulls the Enterprise into a "negative universe" where time seems to flow backwards.