Funny thing is, if you can find a way to explain the existence of 2 different Earth's (Fallout's Wasteland Earth and Metroid's Galactic Federation Headquarters Earth) this Fallout/Metroid could happen. The high radiation around Fallout's Earth causes Samus's ship to crash onto its surface, damaging the majority of her suit in the process. While trying to gather materials and repair her ship and suit, she comes across a Vault-based Settlement, which she becomes part of, trading their help for her advanced technology and knowledge. Utilizing fusion cores and Fallout Earth's plasma technology, she's able to create a basic variation of the 'energy scythes' that the space pirates use (thus her 'lightsaber pike'). In exchange for this weapon to help them fight the dangers of the wasteland, the Vault gives Samus an armored Vault Suit composed of durable composite fibers, which, when combined with a little nano-tech from her Power Armor, allows her to utilize her Arm Cannon.



From here, Samus explores the Wasteland, searching for Vaults and signs of 'alien activity', believing the two are both linked and can lead to an explanation for this version of Earth she is stranded on. Given the numerous experiments that have been discovered going on in Vaults across the Wasteland, Samus can easily guess that this Earth is part of some grand-scale experiment, but the real question she wants answered is:



For what purpose?