Captain Kirk (William Shatner) makes the (questionable!) decision to pursue the mission anyway. Sure enough, the ship is badly damaged while flying into a mysterious pink special effect and has to pull back with their warp drive badly damaged.
While in the galactic barrier, several crew members mysteriously collapse, including the ship’s new psychiatrist Dr. Elizabeth Dehner (Sally Kellerman), and Kirk’s longtime friend, helmsman Gary Mitchell (Gary Lockwood.) Gary’s eyes glow a disturbing silver when he wakes up. Foreshadowing!
The storyline here is a Frankenstein riff; the puny humans reach for knowledge beyond their kin and are duly punished. In this case, though, they are reaching for knowledge not by digging up dead bodies for research. Instead they’re crossing borders and pushing back the, ahem, final frontier.
The consequences of the overreach are also telling. The danger of projecting power across borders is not an enemy attack. It’s not even that you’ll stretch your resources too thin. The danger of exercising power is that one becomes too powerful.