Vault 4 is one of the first vaults established by Vault-Tec and it served as a control for many other vaults.
Cooper Howard shoots a commercial for Vault 4 where he explains that several scientists will be living there for 5 years to test out its feasibility.
When Lucy and Maximus enter a medical facility looking for something to fix his wound, they fall through a trap door and end up in Vault 4.
Lucy discovers that this vault differs significantly from her own because they have had regular contact with the surface.
Many of the current inhabitants are refugees from the surface who came in following the fall of Shady Sands. She meets Birdie, one of the lead refugees.
Lucy and Maximus are told not to go down to Level 12 or ask about it, and that piques Lucy’s interest.
She also realizes that there are several mutated humans in the vault, including the Overseer Benjamin who is a cyclops.
Behind the curtain
While Lucy is excited to be in a vault once again, Maximus is unsettled by how kind everyone else is and accuses them of being in a cult.
The roles reverse soon after though, as Maximus gets a taste of the comfort that comes with living in a vault.
Lucy visits the classroom where she gets a brief history of the formation of the New Californian Republic before the bombs fell.
She then follows the surface dwellers as they prepare for a ritual in the main hall. It turns out to be a worshipping ritual, and the person being worshipped is Moldaver, the Flamemother.
Lucy decides to dig deeper and goes into level 12 and finds evidence of experiments being conducted on some of the people.
When she is caught, she burns the scientist in retaliation before being restrained. Benjamin and Birdie show her a recording from years before.
In it, the scientist from the Vault 4 commercial is panicked, still advocating for scientists to remain in command of vaults despite how catastrophic their experiments have been.
A gulper shows up on the screen and kills the scientists, and then Benjamin and Birdie explain that the original vault dwellers experimented on people from to surface to produce radiation-resistant species.
She is then taken back to the hall to face her punishment, and Maximus steals their fusion core to power his armor and rescue her.
However, she is just banished and after leaving the vault, Lucy convinces Maximus to return the core and leave the armor behind.
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