Fallout 4 is a game usually set in the year 2277 about 200 years after a nuclear holocaust. Where in this universe computers never became advanced and America and the world stayed practically in the 50s for 100 years until 2077 when the world was destroyed. Meanwhile a company called vault tech made vaults designed to save people which they knew the end was inevitable. But the vaults had a secret purpose to test phycological experiments (look into these as these are very interesting and cruel) in most games you play a person who lived in a vault and you leave for good reason. The wasteland (as the outside is referred to) holds many things such as raiders that need no introduction the brotherhood of steel a group of people who want to preserve old world technology (power armor vertiberds plasma and laser weapons) or to some technology hoarders. Super mutants who are test experiments who were supposed to me super soldiers who are faster stronger but are externally in intelligent (a side effect of the drugs the were given) then there is also the enclave which are the last of the U.S. Government and they want to rebuild society without any mutants present (sounds like a noble cause but everyone in the wasteland has developed mutations due to the radiation) deathclaw's cazadors night stalkers rad scorpions yao gui are the animals that have been mutated and I'll let you find those out on your own but overall a very good game you will not regret playing any of them trust me

It's kind of like a different tangent from our own timeline where there was ww3 and nuclear bombs destroyed everything with a futuristic 50s theme. The people who survived did so In underground vaults which was advertised as protection from the bombs but the vaults were secretly used for experimentation and when the vaults opened there was the wasteland.