Age Play With Grandpa

Jenny is off to stay with the man she grew up calling grandpa, her
stepmothers father. She has heard the rumours going around that grandpa
is a very dirty old man, but has never thought much of them. But on her
first night with grandpa she discovers that he is indeed the dirty old man they say he is and he makes Jenny an indecent proposal.

This is a pseudo-incest tale. All characters eighteen or over.