4. “Tomorrow Is Yesterday” (Star Trek, season 1, episode 19)

Col. Fellini (Ed Peck) questions Kirk (William Shatner) in “Tomorrow Is Yesterday.”

The series’ first major foray into time travel—if you put aside the relatively minor three-day rewind in “The Naked Time”—sends a wounded Enterprise back to 1968, where the timeline is corrupted by Kirk’s impulsive decision to beam aboard an Air Force pilot who was chasing the wounded starship. At first, Spock decides that the future might not be affected by the mishap, as he somewhat rudely observes that their captive pilot made no “relevant contribution” to history. But when the Vulcan first officer takes a second look at his history database, he realizes that the pilot’s son became a space exploration hero, so the Enterprise crew can’t just fire the pilot out of the airlock without sweating the consequences. The mess grows from there, but ultimately, Kirk authorizes a plan to slingshot the Enterprise around the sun, a maneuver that conveniently sends the ship slightly backward in time before it goes forward again. That way, the crew can tidily put everything on 1968 Earth back the way it was before returning to their proper time—the first of many times Star Trek would hit the time-travel reset button. (The slingshot trick in particular would be reprised in the similar “back to the 20th century” plot of Star Trek IV: The Voyage Home.) [John Teti]