The Big Mistake Fry Made That Ended Up Saving Everyone In Futurama
For Futurama's Planet Express Crew, saving the world is no big deal. It seems like every other week, the team had to briefly get their troubled lives in order to save Earth from dangers of their own or someone else's making. However, it's rare that any of them achieve a goal so lofty as saving the entire universe. That is, until season 5 episode 8, The Why of Fry.
That episode saw Philip J. Fry get recruited by Leela's pet alien Nibbler on a mission to prevent total universal destruction by their nemesis, the Big Spawn. But why Fry, the most idiotic and bumbling of Planet Express' star-faring ilk? That's because he lacks something called the delta brainwave. Without it, he is simply too stupid to be affected by the Big Spawn's psychic attacks. However, there's an interesting reason why Fry lacks this brainwave, and it has something to do with time travel – and a huge mistake he made in an earlier episode where the crew traveled back to the 20th century.
Fry is the universe's savior because of the grandfather paradox
In the season 4 episode Roswell That Ends Well, the Planet Express Crew accidentally launches themselves back in time to 1940s New Mexico, where they are mistaken for the infamous Roswell alien. By chance, Fry's grandfather Enos is stationed at the Roswell military base at that time, prompting Fry to go visit him despite the possibility of changing the timeline.
Fry becomes obsessed with keeping Enos safe to make sure he continues to exist. However, Fry is an idiot, and the safe house he locks Enos in turns out to be on a testing site for the atomic bomb. Enos is then killed in the ensuing nuclear explosion, and his grandson becomes distraught, believing that he will fade out of existence at any moment. When Fry's would-be grandmother copes with the grief of her fiancé's death by propositioning him, he realizes that she must not be his actual grandma and accepts her advances. The next morning, his co-workers are horrified to learn what he has done. The Professor explains that Fry didn't just sleep with his grandma but also became his own grandfather.
This "past-nastification," as Nibbler refers to it, is what allowed Fry to develop without a delta brainwave. And it just so happens that it made him the only person capable of beating the Brain Spawn. It also allows him to avoid having his mind read by telepaths, therefore helping him defeat the Dark Ones in the movie Into the Wild Green Yonder.