Kirk and his crew are at deadly risk from an alien creature that feeds on the salt in a human body and can take on any form.

Charlie Evans, the sole survivor of a transport crash, is transferred to the Enterprise from a different ship. Why are they so eager to rid themselves of him, though? There's more to Charlie than meets the eye...

While exploring the edge of the galaxy, the Enterprise encounters an energy barrier that gives some crewmen strange powers.

A strange alien substance causes the crew to lose their inhibitions.

The Enterprise picks up a intergalactic conman, Harry Mudd, and three incredibly beautiful women who harbor a dark secret.