Once upon a time, this was a true story.
I think that sentence has potential as an opening line.
And all true stories are far less believable than the fictions they’re based upon.
How about this for an ending:
And they all lived happily ever after, but ‘once upon a time’, if you’ll recall, because now they’re all quite dead.
In the middle, a bunch of stuff happened, and it was much like the time my cat learned to use the coffeemaker and then no one could find a clean spoon, but not exactly like that, because it was something else.