Fallout Just Solved A Huge Mutation Mystery
"Phase 2" of Vault-Tec's plans bring a strange virus into the series canon.

Fallout may be sticking closely to the canon of the original games, but that doesn’t mean that everything will be the same. We’ve already learned about Mr. House’s decoy secret, and apparently, the ending of this season will be different from any of the endings in Fallout: New Vegas. Because of that, any similarity to the games still comes as a complete surprise — and could tease something bigger in the future.
In Season 2 Episode 5 of Fallout, one of the biggest parts of the game finally became canon, meaning we could see an iconic creature in the future.
Warning! Spoilers ahead for Fallout Season 2 Episode 5.
Norm discovers a secret project called “F.E.V.” in Fallout Season 2 Episode 5.
In Fallout Season 2 Episode 5, “The Wrangler,” Norm and the newly-awakened Bud’s Buds venture out of their vault and into Vault-Tec HQ, where Norm hacks into a computer terminal to figure out the true purpose of the vaults. Onscreen, we see the acronym “F.E.V.,” an initialism that’s later revealed to be exactly what fans were hoping it would be: Forced Evolutionary Virus.
F.E.V. is essentially a way to speedrun evolution, a bright green liquid that, when consumed, mutates the user a certain amount. In the Fallout games, this substance is used by the Enclave (the group responsible for the science experiments of Season 1 of the series). They used it to create super mutants, a new subspecies of super-strong (but sterile) creatures who roam the wasteland.
A fan theory suggests that Thaddeus may be turning into a mutant, not a ghoul.
The clues for F.E.V. appearing have been there all along. Remember that weird “cure” Thaddeus took back in Season 1? One major fan theory posits that he actually was exposed to F.E.V., so his “ghoulification” is actually him turning into a mutant. The phrase “super mutant” was even expressly said in the Season 1 finale, when the representative for West-Tek suggests creating “a vault to develop a super-mutant soldier using illegal immigrants.”
So what does this mean for the future of Fallout? F.E.V. and super mutants appear in every single Fallout game, so it doesn’t hint at anything specifically, but this inclusion means that the appearance of a live-action super mutant is a matter of when, not if. For many, Fallout isn’t Fallout without super mutants, so perhaps that’s the big reveal we have to look forward to at the end of the season. Perhaps, Season 3 of Fallout could explore more about this key piece of canon.