Original Star Trek: City on the Edge of Forever.
Dr. McCoy, accidentally injected with an overdose of a massive stimulant, becomes violently paranoid, and jumps through an ancient alien artifact that sends people to the past of their homeworlds. The moment that he does, the entire Federation of Planets ceases to exist; its only remnants are the rest of the landing party, including Kirk and Spock. Realizing that the doctor has changed history for the worse, Kirk and Spock go through the portal after him, landing in Chicago in the 1930s. They take a room in a boarding house while looking for McCoy, and Kirk (predictably) falls in love with the landlady, Edith Keeler (played by Joan Collins, so who could blame him?); Spock, meanwhile, finds that the change to history came when Edith, an extreme pacifist, entered politics, and managed to keep the US out of WWII until the Nazis were unstoppable. (Meanwhile, Edith also finds McCoy, takes him home, and nurses him back to health, not knowing that he was exactly who her "new beau" was looking for.)
At the end, Kirk and Edith are about to go to a movie, when McCoy comes out of the boarding house. Kirk hurries across the street to greet his old friend. Edith stands there for a moment, then comes toward them - as a truck is roaring down the street. McCoy tries to save her, but Kirk, realizing that this is the moment that things changed, holds him back, and Edith is struck and killed by the truck. Closeup on Kirk's agonized face.
MCCOY: My God, Jim! Do you know what you've done?
SPOCK: He knows, Doctor. He knows.
_________________
Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.