My understanding of what an allegory is is that it is a true story where certain elements of the story are represented with symbols.
For example, in the Creation story, Adam and Eve represent our first parents, the man and woman from whom we are all descended. We don’t know what their real names were, or what sort of language they had. The serpent represents the Devil.
Somehow, in what way we don’t know, the Devil communicated with our first mother and induced her to disobey God in some way, which is represented by the taking of the “fruit” (outcome?) of a “forbidden tree” (something was occurring spontaneously that they weren’t supposed to participate in?) . She then caused our first father, her husband (whom we call Adam), to share in the same sin.
When I was younger, I used to think that maybe she tapped a gas well, set it on fire, and then, after she showed the pretty flames to Adam, he started playing with it and accidentally burned down the Garden of Eden, thus explaining the “sword of fire” that evicted them at the end of the story - it also explains why the entire region is desert country today).
The disobedience itself, whatever form that took, is what caused the human race to become estranged from God, which is really what Original Sin is, essentially - the lack of a relationship with God.
As I get older, I realize that whatever our first parents did was much more serious than either taking a piece of fruit or setting a fire - whatever they did was a premeditated act of wilful disobedience towards God; not something that could happen by accident or through thoughtlessness.
The fact that we don’t remember what the sin was isn’t so important, though, as long as we keep in mind that all sin creates a distance between ourselves and God.