Where No Man Has Gone Before

While investigating the fate of the SS Valiant, a ship lost two centuries earlier, the Enterprise crosses a mysterious energy barrier at the edge of the galaxy. The ship is crippled, crew members are killed, and two officers are changed: helmsman Gary Mitchell, Kirk's long-time friend, and psychiatrist Dr Elizabeth Dehner. Mitchell's eyes glow silver as his mental powers grow at a frightening rate, and so does his contempt for ordinary humans. Spock urges Kirk to act before it is too late. Kirk instead tries to maroon Mitchell at the automated station on Delta Vega, but Mitchell breaks free. Kirk must face his friend alone. Dehner, transformed herself, finally sides with Kirk, at the cost of her own life, and Mitchell is buried beneath a rockslide.