Harry and hermione are on their grand tour after finishing eighth/seventh year and having broken up with the Weasleys when they awake to find a 15 month old that looks exactly like Harry did then save for no scar. A medical check shows he has the exact same magical signature Harry had as a child.
The day they go to see off the Child to Hogwarts he disappears back to September 1st 1991. Harry raised himself to be the man he needs to be in the future.

Kind of a reverse time travel story where Harry and Hermione Potter become the new Merlin and Significant Other of.
Actually, this has ME totally confused now.