The Enterprise is en route to the edge of the galaxy, where a barrier of energy lies that has never been penetrated. When the Enterprise reaches the barrier, it is buffeted by intense energy, injuring many on board. First Officer Mitchell and psychological observer Dr. Dehner are affected as well, and it becomes apparent that their latent ESP abilities have been activated by contact with the barrier. The crew must then contend with the rapidly strengthening super-human beings who now consider the other people on board to be an inferior species.
