"The City on the Edge of Forever" airs

"The City on the Edge of Forever" is the penultimate episode of the first season of Star Trek.

It is episode #28, production #28, first broadcast on April 6, 1967. It was repeated on August 31,1967 and marked the last time NBC aired the series on Thursday nights. It was one of the most critically acclaimed episodes of the series and was awarded the 1968 Hugo Award for Best Dramatic Presentation. The other episode with such an honor is the two-parter "The Menagerie". The teleplay is credited to Harlan Ellison, but was controversially rewritten by several hands before filming. It was directed by Joseph Pevney. It guest-stars Joan Collins as Edith Keeler.

The episode involves crew of the Enterprise discovering a portal through space and time, which leads to McCoy accidentally altering history.