A motorcycle racer/stunt rider takes his bike out for a spin and rides past an unlikely machine which sends him back in time where...
He becomes his own great-great granddad.
By banging his great-great grandma.
Bloody heck, that'd be an awkward one at family get-togethers.
"I ever tell you about your relatives from New Mexico?"
"Er... Prob best to leave that branch of the tree unexplored, Grandpa"