Lt. Commander
Gary Mitchell
Gary Lockwood

Dr. Elizabeth Dehner
Sally Kellerman

Doctor Piper
Paul Fix

Lieutenant Lee Kelso
Paul Carr

Yeoman Smith
Andrea Dromm

Lieutenant Alden
Lloyd Haynes

While on a mission to gather data about the force field which surrounds the galaxy, the Enterprise intercepts a distress signal from the flight recorder of S.S. Valiant which had also come across the force field two hundred years previously and had since been listed as missing.

On analysing the memory banks of the recorder, Spock notes many requests for information on E.S.P. ( extra-sensory perception ) and hears the captain of the Valiant ordering the self-destruction of his vessel.

The Enterprise also tries, unsuccessfully, to breach the barrier, resulting in the deaths of nine crewmembers and serious injuries to psychiatrist Dr. Elizabeth Dehner and Lt. Commander Gary Mitchell. As they recover, Mitchell and Dehner start to display an ability to control objects with their minds, including the controls of the Enterprise.

The ship's surgeon, Dr. Piper, believes that they are rapidly evolving into a new form of human at a fantastic rate and, realising the threat to the Enterprise, Spock recommends that they be transported down to a deserted Lithium cracking facility on the planet Delta Vega.

Kirk disagrees at first, as Gary Mitchell is an old Starfleet Academy friend, but is forced into the situation when Mitchell murders a crewmember while escaping from shipboard confinement. Captain Kirk and Dr. Dehner, who is apparently less affected, follow Mitchell down to the planet to try and reason with him but the changes have progressed too far.

He tries to kill Kirk, who is partly protected by the limited powers of Dehner, and in the ensuing battle both mutants are eventually killed. James Kirk, in memory of the friend that he once knew, logs both deaths as being the result of the performance of their duties.