When pre-E3 footage of Fallout 4 showed gameplay that took place before the war, some speculation asked whether this was a prologue to the game - possibly for character creation and as a way to introduce gameplay elements. It seems this is indeed the case. You take control of a family man (or woman) in this world before the wasteland - although the TV hints at the horrors to come.
Once your character's all set up, the alarms sound and so beings an impressive sequence involving a mass exodus to the vault. This cumulates in a simply awesome nuke going off. What happens next, we don't know; what we do know is that the protagonist awakes 200 years later, and probably isn't in a very good mood.