(If you're looking for the recap of part five, it's here.)
The main part of the Clennam family secret is, I think, one of the best secrets in literature, or at least in the literature that I've read. It's big and juicy and very difficult to guess. I for one didn't see it coming when I read the book. Mistaken paternity, after all, is a dime a dozen, but a maternity mix-up? That's a lot trickier to pull off.
Who were Arthur's real parents?
Many years ago, a Mr. Gilbert Clennam put forward his orphan nephew (Arthur's father) as husband for the Mrs. Clennam in the story. They were married. Later, this Mrs. Clennam discovered that her husband had already gone through a form of marriage with another woman, who had borne him a son. Mrs. Clennam, a woman of vindictively self-righteous religiosity, demanded that the child (Arthur) be given into her own custody: if not, she would expose her husband, and bring it about that his uncle cut off his financial support. She got her way, taking possession of the child Arthur, while his true mother went mad and died, and his father went abroad and later died too.
[A word about that "form of marriage" business. I'm not clear on whether Dickens meant that the young couple -- Arthur's father and his real mother -- had actually secretly married, or whether he was just trying to be coy about the fact that they'd slept together. I know which idea I like better -- think of the Mrs. Clennam we know finding out not only that there was a child, but also that her own marriage wasn't valid -- but I don't know which one Dickens intended to convey. We're told that Arthur's real mother acted very guilty when Mrs. Clennam confronted her, but also that she referred to a "desecrated ceremony of marriage" (Mrs. Clennam's words) that they had gone through. Sounds like they may have made vows to each other without benefit of clergy, or something like that. --GRD]
Meanwhile, Gilbert Clennam has heard of the existence of Arthur's true mother; but all he has heard is that she was a girl whom his nephew had loved, but had abandoned in order to marry as his uncle had wished, and that she had subsequently gone mad and died. He has felt remorse at this, and as a kind of recompense has left, in a codicil to his will, a thousand guineas to the youngest daughter of the man who had at one time acted as patron to this girl (i.e. Arthur's true mother); or, if that man had no daughter, to his brother's youngest daughter. [Emphasis Dickens's, not mine. No one uses italics like a Victorian. --GRD]
This man who had acted as patron to Arthur's mother was Frederick Dorrit: he had helped her, in her youth, to be a professional singer. But Frederick Dorrit had had no daughter. The legacy therefore became due to the youngest daughter of his brother: that is to say, to Little Dorrit herself.
And you know the rest: Mrs. Clennam hid the codicil and related papers and later tried to destroy them, but Jeremiah -- "either as giving him a chance to blackmail Mrs. Clennam, or simply for the satisfaction of knowing that he had bested her" -- smuggled them to his brother, and Blandois got them from him and thought he'd try a spot of blackmail on his own account.
Clear as mud? Okay. Here are the things that Andrew Davies changed in the miniseries. First, if I understood correctly, he made Arthur's mother a dancer instead of a singer; I have no idea why. Just so Arthur could find her dancing shoes in the box? Doesn't seem like much of a memento, but whatever. Second, from what Blandois says, Davies seems to have changed Arthur's father's premarital fling (or secret marriage?) into an extramarital fling, so that it was actually adultery. I preferred it the other way, but as far as the story's concerned it doesn't make a great deal of difference, I suppose. Third, he makes Gilbert Clennam the father of Arthur's father, instead of the uncle of Arthur's father. Fourth, he specifies that Arthur's real mother wrote directly to Gilbert Clennam for help, which wasn't in the book.
Fifth -- now, I think this is the biggest change. In the miniseries, we hear that Gilbert Clennam, wanting to recompense Arthur's real mother for her suffering but knowing she was already dead, left money in her honor to a little poor child born in prison on the same day Arthur's mother had died -- Amy Dorrit. And that works fairly well. Truthfully, I can never read all that business about "the youngest daughter of the brother of the patron" without a "Say what?"
BUT -- if I heard everything right -- they actually make it sound in the miniseries as if Gilbert Clennam had picked a poor child at random, and that would be a bigger coincidence than even Dickens ever knew. Personally, I think it works best if you added both reasons together -- to wit, "I can no longer help this poor dead woman, but this patron/landlord of hers has a niece born in poverty the same day she died. I'll leave his niece some money in this woman's honor." It makes so much more sense that I can't imagine why Davies didn't do it that way. Things are already so complex that one more little complexity wouldn't have hurt. Actually, it probably would have simplified everything.
(By the way, in case you're interested in the timeline, all this means that Arthur's real mother died, Amy was born, and Arthur went to China, all around the same time. This actually tallies with the book.)
And finally, Davies has Jeremiah say that he didn't burn the papers because he had no right to -- meaning, I presume, that he could have gotten in legal trouble over it. Which, again, makes sense, but, again, is even stronger if you add it to the reasons Dickens gave (i.e., he had no legal right plus he wanted to get the better of Mrs. Clennam plus he had considered blackmailing her himself).
And there you have it: The dark and tangled (and extremely long) history of the House of Clennam. However, I know we have readers here who know more about Dickens than I do, so if I've missed anything important or gotten anything wrong, please weigh in. And if anyone still has questions, ask away!