3. Star Trek
If you change the past, do you change the future? The award-winning 1967 Star Trek episode called The City on the Edge of Forever answered that time-travel question with an emphatic yes. The Enterprise crew finds a portal through space and time, and McCoy jumps through it to 1930s America. Oops -- McCoy alters the past enough to erase the existence of the Enterprise. And so Kirk and Spock go back in time, to stop McCoy from altering history, and there they meet Joan Collinss Edith. Kirk falls for her, but, well, he cant change history, and therefore he cant save her life. And if he did save her life, her pacifism would have enabled the Nazis to develop a nuclear bomb. Yeah, Edith had to die.
Pictured, an undated scene from the series (not that exact episode), starring (from left) Leonard Nimoy as Mr. Spock, William Shatner as Captain Kirk, DeForest Kelley as Doctor McCoy, and James Doohan as Commander Scott.