In what book...
... do Jamie and Claire tell Jenny and Ian the truth about Claire being a time traveler?
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"How do you know this?"
"That's what I do... I drink, and I know things."
Book #7.. Claire tells the truth to Jenny and family (including Ian, their children, their wives)
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At the point of Crisis and Annihilation, Survival is Victory- Dunkirk
That's when she warns Michael about the French Revolution isn't it?
As for Young Ian, he sort of works it out from the 1960s character that goes back to try and prevent the fate of the Native Americans, doesn't he, and Claire confirms it for him. I just remember him saying he never believed that she was a fairy.
(aha, spoiler tags, got it)
Did young Ian not know before that?
Geilis abducted him and transported him to the cave in the West Indies(or somewhere near) where she was going to use him as a human sacrifice to travel through time.
No, he didn't.
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At the point of Crisis and Annihilation, Survival is Victory- Dunkirk
I don't think he was entirely conscious for all of that, and when he was awake, I don't think he understood what was going on.
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"How do you know this?"
"That's what I do... I drink, and I know things."