Has any one individual ever saved the "world" so to speak?

"It happens all the time in movies, but has there ever been a time throughout history where the day was saved by one individual. One clinching moment where they returned in the last scene to stop the world as everyone knew it from tumbling down around them?"

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That guy(s) who swam through radioctive water during and prevented an even bigger explosion in the time of the Chernobyl crisis.... They saved half of Europe, including my own homeland, from becoming uninhabitable and they did it without a flashlight in the water. There were over 50 million people living in Ukraine at that time and over 35 million in Poland. Many other countries. It is easily possible it would've made more than 100 million people refugees.
These men were Valeri Bezpalov, Boris Baranov and Alexei Ananenko. I personally own them my graditude.

I'd say this guy is the closet I am aware of.

On September 26, 1983, just three weeks after the Soviet military had shot down Korean Air Lines Flight 007, Petrov was the duty officer at the command center for the Oko nuclear early-warning system when the system reported that a missile had been launched from the United States, followed by up to five more. Petrov judged the reports to be a false alarm,[2]and his decision is credited with having prevented an erroneous retaliatory nuclear attack on the United States and its NATO allies that could have resulted in large-scale nuclear war.

Maybe not as dramatic as a Hollywood movie, but Jonas Salk and his Polio vaccine saved the lives of millions of children around the world.

Clair Patterson successfully fought the lead industry to make lawmakers realize it was a toxic pollutant that affected people psychologically by increasing violent tendencies, along with a myriad of other health concerns.

Norman Borlaug - https://en.m.wikipedia.org/wiki/Norman_Borlaug

Borlaug is often called "the father of the Green Revolution",[5][6] and is credited with saving over a billion people worldwide from starvation.[7][8][9][10]According to Jan Douglas, executive assistant to the president of the World Food Prize Foundation, the source of this number is Gregg Easterbrook's 1997 article "Forgotten Benefactor of Humanity", the article states that the "form of agriculture that Borlaug preaches may have prevented a billion deaths."[11] He was awarded the Nobel Peace Prize in 1970 in recognition of his contributions to world peace through increasing food supply.