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The shooting script (mostly for Google’s benefit)

DAN
So I’m building a time machine.
LISA
Again?
DAN
Wait. What do you-?
LISA
You built one like a year ago.
DAN
I don’t remember that.
LISA
You started going back in time and messing things up. Stepped on a butterfly in the Jurassic, became your own great-grandfather. You would travel back in time and leave the toilet seat up just to get your past self in trouble with me.
DAN
But-?
LISA
Once the robots took over you realized you were playing with forces you didn’t understand. So you went back and convinced yourself not to build the machine in the first place. That pretty much put everything back to normal. Before you left you told me never to let you build another time machine. You said the future of the Universe depended on it.
DAN
Why don’t I remember any of this?
LISA
Because it never actually happened. The version of you that remembered what happened never came back from the last trip to the past because you prevented yourself from inventing the device that would have brought you back.
DAN
So who am I?
LISA
You’re the alternate version that never invented anything.
DAN
What happened to the other one?
LISA
I guess he disappeared. Along with the robot overlords.
DAN
What was he like?
LISA
(dreamily)
Oh, he was so dashing. And idealistic. Always running around trying to save the world.
DAN
But he ruined it.
LISA
Yeah… But I’ll always love him more.
DAN
Which one of us was better in bed?
LISA
(immediately)
The other one.
DAN
Wow, that’s depressing.
Dan looks crushed.
Lisa cracks up.
LISA
I’m sorry. I can’t keep this up.
DAN
What?
LISA
I was just kidding. You never made a time machine.
DAN
That was mean!
LISA
(still laughing)
I’m sorry. Just don’t go messing with the space time continuum or you could end up with a girlfriend who loves your alternate self more.
DAN
I’ll be careful.