A brother and sister grow up best friends as USMC officer's children, fall in love, get married, and have a family. All this time they are being taught how to become spies for their country.
Allie can't have any more babies, even though she and husband John would like another. In a moment of delight as he fills her pussy, she has the idea of getting her son to fuck her eldest daughter pregnant. Perhaps, she reasons, if she were to get him addicted to bareback sex and then somehow contrived to get him up close and personal with Danni, maybe nature would take its course? There's another member of the family, Rachel, who hasn't yet reached menarche, as far as anyone knows...
A father, me, and son, Barney, have to get across to daughter/sister, Midge, the dangers awaiting young girls when they want to start dating. Their warnings must be effective, because young Midge never does go out with the young man she originally fancied, preferring to stay home, where Barney and I drive home the lesson - time after time. You could say, she gets the point.