An alien device on a primitive planet erases Captain Kirk's memory, and he begins a new life with the planet's indigenous people.

Invisible "time-accelerated" aliens take over the Enterprise and attempt to abduct the crew for use as "genetic stock".

The Enterprise encounters two duo-chromatic and mutually belligerent aliens who put the ship in the middle of their old conflict.

A race of overpopulated aliens abduct Captain Kirk to solve their problem.

Captain Kirk races against time to acquire plague-fighting minerals from a world in the midst of a civil uprising against a grievous social class disparity.